Reland of "Expose events in workers as per latest DOM specification"

Source/platform/EventDispatchForbiddenScope.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 EventDispatchForbiddenScope_h
 #define EventDispatchForbiddenScope_h
 
 #include "platform/PlatformExport.h"
 #include "wtf/Assertions.h"
 #include "wtf/MainThread.h"
 #include "wtf/TemporaryChange.h"
 
 namespace blink {
 
 #if ENABLE(ASSERT)
 
 class EventDispatchForbiddenScope {
 public:
     EventDispatchForbiddenScope()
     {
-        if (!isMainThread())
-            return;
+        ASSERT(isMainThread());
         ++s_count;
     }
 
     ~EventDispatchForbiddenScope()
     {
-        if (!isMainThread())
-            return;
+        ASSERT(isMainThread());
         ASSERT(s_count);
         --s_count;
     }
 
     static bool isEventDispatchForbidden()
     {
         if (!isMainThread())
             return false;
         return s_count;
     }
 
     class AllowUserAgentEvents {
     public:
         AllowUserAgentEvents()
             : m_change(s_count, 0)
         {
+            ASSERT(isMainThread());
         }
 
         ~AllowUserAgentEvents()
         {
             ASSERT(!s_count);
         }
 
         TemporaryChange<unsigned> m_change;
     };
 
@@ skipping 11 matching lines @@
     public:
         AllowUserAgentEvents() { }
     };
 };
 
 #endif // ENABLE(ASSERT)
 
 } // namespace blink
 
 #endif // EventDispatchForbiddenScope_h