Use conditionals and hooks_os for download_from_google_storage hooks

This should be a no-op, and is part of a migration path
away from hooks_os.

It's also one of the smallest experiments with conditionals
in chromium/src.

Bug: 570091
Change-Id: I9d582a6703c64a4ab37bb063ebebe85a7b41b4b5
Reviewed-on: https://chromium-review.googlesource.com/671383
Reviewed-by: Michael Moss <mmoss@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Michael Moss <mmoss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#502706}
diff --git a/DEPS b/DEPS
index f47b6ca..b1702b4 100644
--- a/DEPS
+++ b/DEPS
@@ -664,6 +664,7 @@
   {
     'name': 'gn_win',
     'pattern': '.',
+    'condition': 'host_os == "win"',
     'action': [ 'python',
                 'src/third_party/depot_tools/download_from_google_storage.py',
                 '--no_resume',
@@ -676,6 +677,7 @@
   {
     'name': 'gn_mac',
     'pattern': '.',
+    'condition': 'host_os == "mac"',
     'action': [ 'python',
                 'src/third_party/depot_tools/download_from_google_storage.py',
                 '--no_resume',
@@ -688,6 +690,7 @@
   {
     'name': 'gn_linux64',
     'pattern': '.',
+    'condition': 'host_os == "linux"',
     'action': [ 'python',
                 'src/third_party/depot_tools/download_from_google_storage.py',
                 '--no_resume',
@@ -701,6 +704,7 @@
   {
     'name': 'clang_format_win',
     'pattern': '.',
+    'condition': 'host_os == "win"',
     'action': [ 'python',
                 'src/third_party/depot_tools/download_from_google_storage.py',
                 '--no_resume',
@@ -713,6 +717,7 @@
   {
     'name': 'clang_format_mac',
     'pattern': '.',
+    'condition': 'host_os == "mac"',
     'action': [ 'python',
                 'src/third_party/depot_tools/download_from_google_storage.py',
                 '--no_resume',
@@ -725,6 +730,7 @@
   {
     'name': 'clang_format_linux',
     'pattern': '.',
+    'condition': 'host_os == "linux"',
     'action': [ 'python',
                 'src/third_party/depot_tools/download_from_google_storage.py',
                 '--no_resume',
@@ -738,6 +744,7 @@
   {
     'name': 'orderfiles_win',
     'pattern': '.',
+    'condition': 'host_os == "win"',
     'action': [ 'python',
                 'src/third_party/depot_tools/download_from_google_storage.py',
                 '--no_resume',
@@ -751,6 +758,7 @@
   {
     'name': 'luci-go_win',
     'pattern': '.',
+    'condition': 'host_os == "win"',
     'action': [ 'python',
                 'src/third_party/depot_tools/download_from_google_storage.py',
                 '--no_resume',
@@ -763,6 +771,7 @@
   {
     'name': 'luci-go_mac',
     'pattern': '.',
+    'condition': 'host_os == "mac"',
     'action': [ 'python',
                 'src/third_party/depot_tools/download_from_google_storage.py',
                 '--no_resume',
@@ -775,6 +784,7 @@
   {
     'name': 'luci-go_linux',
     'pattern': '.',
+    'condition': 'host_os == "linux"',
     'action': [ 'python',
                 'src/third_party/depot_tools/download_from_google_storage.py',
                 '--no_resume',
@@ -787,6 +797,7 @@
   {
     'name': 'drmemory',
     'pattern': '.',
+    'condition': 'host_os == "win"',
     'action': [ 'python',
                 'src/third_party/depot_tools/download_from_google_storage.py',
                 '--no_resume',
@@ -811,6 +822,7 @@
   {
     'name': 'apache_win32',
     'pattern': '\\.sha1',
+    'condition': 'host_os == "win"',
     'action': [ 'python',
                 'src/third_party/depot_tools/download_from_google_storage.py',
                 '--no_resume',
@@ -855,6 +867,7 @@
   {
     'name': 'node_linux64',
     'pattern': '.',
+    'condition': 'host_os == "linux"',
     'action': [ 'python',
                 'src/third_party/depot_tools/download_from_google_storage.py',
                 '--no_resume',
@@ -868,6 +881,7 @@
   {
     'name': 'node_mac',
     'pattern': '.',
+    'condition': 'host_os == "mac"',
     'action': [ 'python',
                 'src/third_party/depot_tools/download_from_google_storage.py',
                 '--no_resume',
@@ -881,6 +895,7 @@
   {
     'name': 'node_win',
     'pattern': '.',
+    'condition': 'host_os == "win"',
     'action': [ 'python',
                 'src/third_party/depot_tools/download_from_google_storage.py',
                 '--no_resume',