Avoid sending notifications when the bitmap data in history has not changed

base/memory/ref_counted_memory.h

 // Copyright (c) 2012 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 BASE_MEMORY_REF_COUNTED_MEMORY_H_
 #define BASE_MEMORY_REF_COUNTED_MEMORY_H_
 
 #include <string>
 #include <vector>
 
 #include "base/base_export.h"
 #include "base/compiler_specific.h"
 #include "base/memory/ref_counted.h"
 
 namespace base {
 
 // A generic interface to memory. This object is reference counted because one
 // of its two subclasses own the data they carry, and we need to have
 // heterogeneous containers of these two types of memory.
 class BASE_EXPORT RefCountedMemory
     : public base::RefCountedThreadSafe<RefCountedMemory> {
  public:
   // Retrieves a pointer to the beginning of the data we point to. If the data
   // is empty, this will return NULL.
   virtual const unsigned char* front() const = 0;
 
   // Size of the memory pointed to.
   virtual size_t size() const = 0;
 
+  // Returns true if |other| is byte for byte equal.
+  bool Equals(const scoped_refptr<RefCountedMemory>& other) const;
+
  protected:
   friend class base::RefCountedThreadSafe<RefCountedMemory>;
   RefCountedMemory();
   virtual ~RefCountedMemory();
 };
 
 // An implementation of RefCountedMemory, where the ref counting does not
 // matter.
 class BASE_EXPORT RefCountedStaticMemory : public RefCountedMemory {
  public:
@@ skipping 66 matching lines @@
   virtual ~RefCountedString();
 
   std::string data_;
 
   DISALLOW_COPY_AND_ASSIGN(RefCountedString);
 };
 
 }  // namespace base
 
 #endif  // BASE_MEMORY_REF_COUNTED_MEMORY_H_