Fix flaky crashes

Source/core/frame/PlatformEventDispatcher.cpp

 // 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.
 
 #include "config.h"
 #include "core/frame/PlatformEventDispatcher.h"
 
 #include "core/frame/PlatformEventController.h"
 #include "wtf/TemporaryChange.h"
 
 namespace blink {
 
 PlatformEventDispatcher::PlatformEventDispatcher()
     : m_isDispatching(false)
     , m_isListening(false)
 {
 }
 
 void PlatformEventDispatcher::addController(PlatformEventController* controller)
 {
     ASSERT(controller);
-    ASSERT(!m_controllers.contains(controller));
+    // TODO: If we can avoid to register a same controller twice, we can change
+    // this 'if' to ASSERT.
+    if (m_controllers.contains(controller))
+        return;
 
     m_controllers.add(controller);
+
     if (!m_isListening) {
         startListening();
         m_isListening = true;
     }
 }
 
 void PlatformEventDispatcher::removeController(PlatformEventController* controller)
 {
     ASSERT(m_controllers.contains(controller));
 
@@ skipping 28 matching lines @@
 }
 
 DEFINE_TRACE(PlatformEventDispatcher)
 {
 #if ENABLE(OILPAN)
     visitor->trace(m_controllers);
 #endif
 }
 
 } // namespace blink