content: convert zoom notifications to observer usage

content/public/browser/host_zoom_map.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 CONTENT_PUBLIC_BROWSER_HOST_ZOOM_MAP_H_
 #define CONTENT_PUBLIC_BROWSER_HOST_ZOOM_MAP_H_
 
 #include <map>
 #include <string>
 #include <vector>
 
 #include "base/basictypes.h"
+#include "base/callback.h"
 #include "content/common/content_export.h"
 
 namespace content {
 
 class BrowserContext;
 class ResourceContext;
 
 // Maps hostnames to custom zoom levels.  Written on the UI thread and read on
 // any thread.  One instance per browser context. Must be created on the UI
 // thread, and it'll delete itself on the UI thread as well.
@@ skipping 20 matching lines @@
   // the level matches the current default zoom level, the host is erased
   // from the saved preferences; otherwise the new value is written out.
   //
   // This should only be called on the UI thread.
   virtual void SetZoomLevel(const std::string& host, double level) = 0;
 
   // Get/Set the default zoom level for pages that don't override it.
   virtual double GetDefaultZoomLevel() const = 0;
   virtual void SetDefaultZoomLevel(double level) = 0;;
 
+  typedef base::Callback<void(const std::string&)> ZoomLevelChangedCallback;
+
+  // Add and remove zoom level changed callbacks.
+  virtual void AddZoomLevelChangedCallback(
+      const ZoomLevelChangedCallback& callback) = 0;
+  virtual void RemoveZoomLevelChangedCallback(
+      const ZoomLevelChangedCallback& callback) = 0;
+
  protected:
   virtual ~HostZoomMap() {}
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_HOST_ZOOM_MAP_H_