Annotate the benign race in ScriptForbiddenScope::isScriptForbidden().

third_party/WebKit/Source/platform/ScriptForbiddenScope.h

Index: third_party/WebKit/Source/platform/ScriptForbiddenScope.h
diff --git a/third_party/WebKit/Source/platform/ScriptForbiddenScope.h b/third_party/WebKit/Source/platform/ScriptForbiddenScope.h
index 4aa19b5ff5e1ef72fd95d04ea849913d9a2bc385..df11f5c33cc362b17f389f0e9aff7dd47d9cce6f 100644
--- a/third_party/WebKit/Source/platform/ScriptForbiddenScope.h
+++ b/third_party/WebKit/Source/platform/ScriptForbiddenScope.h
@@ -8,6 +8,7 @@
 #include "platform/PlatformExport.h"
 #include "wtf/Allocator.h"
 #include "wtf/AutoReset.h"
+#include "wtf/DynamicAnnotations.h"
 #include "wtf/Optional.h"
 
 namespace blink {
@@ -52,6 +53,9 @@ class PLATFORM_EXPORT ScriptForbiddenScope final {
     // Check s_scriptForbiddenCount first to avoid any calls to isMainThread()
     // since under normal situations where we check this (ex. inside
     // V8ScriptRunner) the value should always be zero.
+    WTF_ANNOTATE_BENIGN_RACE(&s_scriptForbiddenCount,

[Alexander Potapenko, 2017/03/17 10:14:50]

Please no.
WTF_ANNOTATE_BENIGN_RACE is a beast we need to get rid of, not let it grow.
If you're reading something from several threads, these reads must be
synchronized.

+                             "Safe to racy read s_scriptForbiddenCount since "
+                             "the value only matters on the main thread.");
     return s_scriptForbiddenCount && isMainThread();

[Alexander Potapenko, 2017/03/17 10:14:50]

If the value only matters on the main thread, how about we read it on the main
thread only?
Putting isMainThread() on the first place should solve the problem:

  return isMainThread() && s_scriptForbiddenCount;

If it does not, someone might be modifying s_scriptForbiddenCount from another
thread.

   }