Add a WebDocument::watchCssSelectors(selectors)

Source/wtf/WeakPtr.h

 /*
  * Copyright (C) 2013 Google, Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 69 matching lines @@
 #endif
 };
 
 template<typename T>
 class WeakPtr {
     WTF_MAKE_FAST_ALLOCATED;
 public:
     WeakPtr() { }
     WeakPtr(PassRefPtr<WeakReference<T> > ref) : m_ref(ref) { }
 
-    T* get() const { return m_ref->get(); }
+    T* get() const { return m_ref ? m_ref->get() : 0; }
 
 private:
     RefPtr<WeakReference<T> > m_ref;
 };
 
 template<typename T>
 class WeakPtrFactory {
     WTF_MAKE_NONCOPYABLE(WeakPtrFactory<T>);
     WTF_MAKE_FAST_ALLOCATED;
 public:
@@ skipping 22 matching lines @@
     RefPtr<WeakReference<T> > m_ref;
 };
 
 } // namespace WTF
 
 using WTF::WeakPtr;
 using WTF::WeakPtrFactory;
 using WTF::WeakReference;
 
 #endif