Revert "Remove WaitSyncTokenCHROMIUM command"

This reverts commit c19106d109d4d36871c11bfb5a683d222583dd15.

Reason for revert: Causing multiple mac gpu fyi failures crbug.com/895984

Original change's description:
> Remove WaitSyncTokenCHROMIUM command
> 
> After migrating InProcessCommandBuffer to use GPU scheduler, most tasks
> are scheduled after their sync token dependencies are satisifed (see
> crrev.com/c/1157874).
> 
> The one exception was the WaitSyncToken IPC used by ReturnFrontBuffer
> for pepper, which specifies a sync token, and waits while handling the
> message.
> 
> Change ReturnFrontBuffer to contain the sync token, and use it to
> specify the dependency to the scheduler.
> 
> Remove WaitSyncTokenCHROMIUM command, since sync token dependencies are
> specified as task metadata in all cases.
> 
> Make other cleanups such as removing unnecessary sync token tests, and
> make sure the sync token code paths are not used where unsupported by
> using NOTREACHED.
> 
> Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
> Change-Id: Ieee4f6e2427a59a4e0c4b3c983cb489741241272
> Bug: 778753
> Reviewed-on: https://chromium-review.googlesource.com/c/1168155
> Reviewed-by: Bo <boliu@chromium.org>
> Reviewed-by: Daniel Cheng <dcheng@chromium.org>
> Reviewed-by: Antoine Labour <piman@chromium.org>
> Commit-Queue: Sunny Sachanandani <sunnyps@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#599849}

TBR=dcheng@chromium.org,boliu@chromium.org,sunnyps@chromium.org,piman@chromium.org

Change-Id: I87347c05dded27955410b08e40f37388a484d5f9
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 778753, 895984
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Reviewed-on: https://chromium-review.googlesource.com/c/1284394
Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Commit-Queue: Sunny Sachanandani <sunnyps@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#600102}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 063f6d2a2897dcfb9afc3ee80cbb78ef87e9fe2a
diff --git a/proxy/ppapi_command_buffer_proxy.cc b/proxy/ppapi_command_buffer_proxy.cc
index ab29b16..90f4147 100644
--- a/proxy/ppapi_command_buffer_proxy.cc
+++ b/proxy/ppapi_command_buffer_proxy.cc
@@ -171,7 +171,7 @@
 }
 
 void PpapiCommandBufferProxy::SetLock(base::Lock*) {
-  NOTREACHED();
+  NOTIMPLEMENTED();
 }
 
 void PpapiCommandBufferProxy::EnsureWorkVisible() {
@@ -206,23 +206,22 @@
 }
 
 bool PpapiCommandBufferProxy::IsFenceSyncReleased(uint64_t release) {
-  NOTREACHED();
+  NOTIMPLEMENTED();
   return false;
 }
 
 void PpapiCommandBufferProxy::SignalSyncToken(const gpu::SyncToken& sync_token,
                                               base::OnceClosure callback) {
-  NOTREACHED();
+  NOTIMPLEMENTED();
 }
 
-// Pepper plugin does not expose or call WaitSyncTokenCHROMIUM.
-void PpapiCommandBufferProxy::WaitSyncToken(const gpu::SyncToken& sync_token) {
-  NOTREACHED();
+void PpapiCommandBufferProxy::WaitSyncTokenHint(
+    const gpu::SyncToken& sync_token) {
+  // TODO(sunnyps): Forward sync token dependency hints to the renderer.
 }
 
 bool PpapiCommandBufferProxy::CanWaitUnverifiedSyncToken(
     const gpu::SyncToken& sync_token) {
-  NOTREACHED();
   return false;
 }
 
@@ -233,13 +232,13 @@
 
 void PpapiCommandBufferProxy::CreateGpuFence(uint32_t gpu_fence_id,
                                              ClientGpuFence source) {
-  NOTREACHED();
+  NOTIMPLEMENTED();
 }
 
 void PpapiCommandBufferProxy::GetGpuFence(
     uint32_t gpu_fence_id,
     base::OnceCallback<void(std::unique_ptr<gfx::GpuFence>)> callback) {
-  NOTREACHED();
+  NOTIMPLEMENTED();
 }
 
 void PpapiCommandBufferProxy::SetGpuControlClient(gpu::GpuControlClient*) {
diff --git a/proxy/ppapi_command_buffer_proxy.h b/proxy/ppapi_command_buffer_proxy.h
index 8d4d23c..28588f2 100644
--- a/proxy/ppapi_command_buffer_proxy.h
+++ b/proxy/ppapi_command_buffer_proxy.h
@@ -76,7 +76,7 @@
   bool IsFenceSyncReleased(uint64_t release) override;
   void SignalSyncToken(const gpu::SyncToken& sync_token,
                        base::OnceClosure callback) override;
-  void WaitSyncToken(const gpu::SyncToken& sync_token) override;
+  void WaitSyncTokenHint(const gpu::SyncToken& sync_token) override;
   bool CanWaitUnverifiedSyncToken(const gpu::SyncToken& sync_token) override;
 
  private: