Disable runtime feature CompositeOpaqueFixedPosition

This patch fixed the missing part from the previous patch
https://crrev.com/c/939585 to completely disable the feature.

Bug: 814673
Change-Id: Id14a32351d6153221aabf36dec506533ad07b437
Reviewed-on: https://chromium-review.googlesource.com/941262
Commit-Queue: Yi Gu <yigu@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#539830}(cherry picked from commit 5770b54a09c563cf56d7d2aaf7cdda80c59e1be1)
Reviewed-on: https://chromium-review.googlesource.com/941402
Cr-Commit-Position: refs/branch-heads/3325@{#624}
Cr-Branched-From: bc084a8b5afa3744a74927344e304c02ae54189f-refs/heads/master@{#530369}
diff --git a/third_party/WebKit/Source/platform/runtime_enabled_features.json5 b/third_party/WebKit/Source/platform/runtime_enabled_features.json5
index 721a79a..74fbae9 100644
--- a/third_party/WebKit/Source/platform/runtime_enabled_features.json5
+++ b/third_party/WebKit/Source/platform/runtime_enabled_features.json5
@@ -174,7 +174,7 @@
     {
       name: "CompositeOpaqueFixedPosition",
       settable_from_internals: true,
-      status: "stable",
+      status: "experimental",
     },
     {
       name: "CompositeOpaqueScrollers",