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..dcc9e4d7cdff34a8f5693c159e5635620a75a00f 100644
--- a/content/browser/host_zoom_map_impl.h
+++ b/content/browser/host_zoom_map_impl.h
@@ -10,6 +10,7 @@
 #include <vector>
 
 #include "base/compiler_specific.h"
+#include "base/observer_list.h"
 #include "base/sequenced_task_runner_helpers.h"
 #include "base/supports_user_data.h"
 #include "base/synchronization/lock.h"
@@ -57,8 +58,19 @@ class CONTENT_EXPORT HostZoomMapImpl : public NON_EXPORTED_BASE(HostZoomMap),
                        const NotificationDetails& details) OVERRIDE;
 
  private:
+  friend class HostZoomMap::Observer;
+
   typedef std::map<std::string, double> HostZoomLevels;
 
+  // Add and remove observers. Adding or removing multiple times has no effect.
+  // The order in which notifications are sent to observers is undefined.
+  // Clients must be sure to remove the observer before they go away.
+  void AddObserver(HostZoomMap::Observer* observer);
+  void RemoveObserver(HostZoomMap::Observer* observer);
+
+  // A list of observers. Weak references.
+  ObserverList<HostZoomMap::Observer> observers_;
+
   // Copy of the pref data, so that we can read it on the IO thread.
   HostZoomLevels host_zoom_levels_;
   double default_zoom_level_;