content: convert zoom notifications to observer usage

content/browser/host_zoom_map_impl.h

Index: content/browser/host_zoom_map_impl.h
diff --git a/content/browser/host_zoom_map_impl.h b/content/browser/host_zoom_map_impl.h
index 7840188c0d747399cd59fbb42d0871237aafe74f..4e6eff4d626cb3aaaee1f1bea22a7d07bd5cf8ab 100644
--- a/content/browser/host_zoom_map_impl.h
+++ b/content/browser/host_zoom_map_impl.h
@@ -34,6 +34,10 @@ class CONTENT_EXPORT HostZoomMapImpl : public NON_EXPORTED_BASE(HostZoomMap),
   virtual void SetZoomLevel(const std::string& host, double level) OVERRIDE;
   virtual double GetDefaultZoomLevel() const OVERRIDE;
   virtual void SetDefaultZoomLevel(double level) OVERRIDE;
+  virtual void AddZoomLevelChangedCallback(
+      const ZoomLevelChangedCallback& callback) OVERRIDE;
+  virtual void RemoveZoomLevelChangedCallback(
+      const ZoomLevelChangedCallback& callback) OVERRIDE;
 
   // Returns the temporary zoom level that's only valid for the lifetime of
   // the given WebContents (i.e. isn't saved and doesn't affect other
@@ -59,6 +63,9 @@ class CONTENT_EXPORT HostZoomMapImpl : public NON_EXPORTED_BASE(HostZoomMap),
  private:
   typedef std::map<std::string, double> HostZoomLevels;
 
+  // Callbacks called when zoom level changes.
+  std::vector<ZoomLevelChangedCallback> zoom_level_changed_callbacks_;
+
   // Copy of the pref data, so that we can read it on the IO thread.
   HostZoomLevels host_zoom_levels_;
   double default_zoom_level_;