content: convert zoom notifications to observer usage

content/public/browser/host_zoom_map.h

Index: content/public/browser/host_zoom_map.h
diff --git a/content/public/browser/host_zoom_map.h b/content/public/browser/host_zoom_map.h
index 1e32e79383702721f3128efef30a2e0fff8f82eb..c399f699aec489aa3a1d524aab15b6ab08d120ce 100644
--- a/content/public/browser/host_zoom_map.h
+++ b/content/public/browser/host_zoom_map.h
@@ -10,6 +10,7 @@
 #include <vector>
 
 #include "base/basictypes.h"
+#include "base/callback.h"
 #include "content/common/content_export.h"
 
 namespace content {
@@ -50,6 +51,14 @@ class HostZoomMap {
   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() {}
 };