Add onChanged events to the extension settings API, both from sync and between

chrome/browser/extensions/extension_setting_changes.cc

+// Copyright (c) 2011 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 "chrome/browser/extensions/extension_setting_changes.h"
+
+#include "base/json/json_writer.h"
+
+void ExtensionSettingChanges::Builder::AppendChange(
+    const std::string& key, Value* old_value, Value* new_value) {
+  DictionaryValue* change = new DictionaryValue();
+  changes_.Append(change);
+
+  change->SetString("key", key);
+  if (old_value) {
+    change->Set("oldValue", old_value);
+  }
+  if (new_value) {
+    change->Set("newValue", new_value);
+  }
+}
+
+ExtensionSettingChanges ExtensionSettingChanges::Builder::Build() {
+  return ExtensionSettingChanges(&changes_);
+}
+
+ExtensionSettingChanges::ExtensionSettingChanges(ListValue* changes)
+    : inner_(new Inner()) {
+  inner_->changes_.Swap(changes);
+}
+
+ExtensionSettingChanges::~ExtensionSettingChanges() {}
+
+std::string ExtensionSettingChanges::ToJson() const {
+  std::string changes_json;
+  base::JSONWriter::Write(&inner_->changes_, false, &changes_json);
+  return changes_json;
+}