Improve ScriptForbiddenScope handling in cross-threaded code.

third_party/WebKit/Source/platform/ScriptForbiddenScope.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef ScriptForbiddenScope_h
 #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();
 
     class PLATFORM_EXPORT AllowUserAgentScript {
     public:
         AllowUserAgentScript();
         ~AllowUserAgentScript();
     private:
         Optional<TemporaryChange<unsigned>> m_change;
     };
 
     static void enter();
     static void exit();
     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