Make the hcsm and its providers communicate via an observer interface.

chrome/browser/content_settings/content_settings_observer.h

+// 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.
+
+#ifndef CHROME_BROWSER_CONTENT_SETTINGS_CONTENT_SETTINGS_OBSERVER_H_
+#define CHROME_BROWSER_CONTENT_SETTINGS_CONTENT_SETTINGS_OBSERVER_H_
+
+#include <string>
+
+#include "chrome/browser/content_settings/content_settings_pattern.h"
+#include "chrome/common/content_settings_types.h"
+
+namespace content_settings {
+
+class Observer {
+ public:
+  virtual void OnContentSettingChanged(
+      ContentSettingsPattern primary_pattern,
+      ContentSettingsPattern secondary_pattern,
+      ContentSettingsType content_type,
+      std::string resource_identifier) = 0;
+};
+
+}  // namespace content_settings
+
+#endif  // CHROME_BROWSER_CONTENT_SETTINGS_CONTENT_SETTINGS_OBSERVER_H_