Improve ScriptForbiddenScope handling in cross-threaded code.

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 9c3ba7e8f5dda74bb6ab5445c065f6fecf9b7b77..db7bc6b3e1e62776f0f047c092b254ff61aa839d 100644
--- a/third_party/WebKit/Source/platform/ScriptForbiddenScope.h
+++ b/third_party/WebKit/Source/platform/ScriptForbiddenScope.h
@@ -6,13 +6,16 @@
 #define ScriptForbiddenScope_h
 
 #include "platform/PlatformExport.h"
-#include "wtf/Assertions.h"
+#include "wtf/Allocator.h"
 #include "wtf/Optional.h"
 #include "wtf/TemporaryChange.h"
 
 namespace blink {
 
+// Scoped disabling of script execution on the main thread,
+// and only to be used by the main thread.
 class PLATFORM_EXPORT ScriptForbiddenScope {
+    STACK_ALLOCATED();
 public:
     ScriptForbiddenScope();
     ~ScriptForbiddenScope();
@@ -30,6 +33,20 @@ public:
     static bool isScriptForbidden();
 };
 
+// Scoped disabling of script execution on the main thread,
+// if called on the main thread.
+//
+// No effect when used by from other threads -- simplifies
+// call sites that might be used by multiple threads to have
+// this scope object perform the is-main-thread check on
+// its behalf.
+class PLATFORM_EXPORT ScriptForbiddenIfMainThreadScope {
+    STACK_ALLOCATED();
+public:
+    ScriptForbiddenIfMainThreadScope();
+    ~ScriptForbiddenIfMainThreadScope();
+};
+
 } // namespace blink
 
 #endif // ScriptForbiddenScope_h