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

base/memory/ref_counted_memory.cc

 // 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.
 
 #include "base/memory/ref_counted_memory.h"
 
 #include "base/logging.h"
 
 namespace base {
 
+bool RefCountedMemory::Equals(
+    const scoped_refptr<RefCountedMemory>& other) const {
+  return other.get() &&
+         size() == other->size() &&
+         (memcmp(front(), other->front(), size()) == 0);
+}
+
 RefCountedMemory::RefCountedMemory() {}
 
 RefCountedMemory::~RefCountedMemory() {}
 
 const unsigned char* RefCountedStaticMemory::front() const {
   return data_;
 }
 
 size_t RefCountedStaticMemory::size() const {
   return length_;
@@ skipping 40 matching lines @@
 const unsigned char* RefCountedString::front() const {
   return data_.empty() ? NULL :
          reinterpret_cast<const unsigned char*>(data_.data());
 }
 
 size_t RefCountedString::size() const {
   return data_.size();
 }
 
 }  //  namespace base