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..2358428706cb2802eabbcc0493b1cd2ec7283d70 100644
--- a/content/public/browser/host_zoom_map.h
+++ b/content/public/browser/host_zoom_map.h
@@ -15,6 +15,7 @@
 namespace content {
 
 class BrowserContext;
+class HostZoomMapImpl;
 class ResourceContext;
 
 // Maps hostnames to custom zoom levels.  Written on the UI thread and read on
@@ -25,6 +26,29 @@ class HostZoomMap {
   CONTENT_EXPORT static HostZoomMap* GetForBrowserContext(
       BrowserContext* browser_context);
 
+  class Observer {

[jam, 2013/01/24 01:28:35]

nit: per c++ style guide, and convention, prefer to put every class into a
separate file. this avoids having to include this header in other headers that
just want to implement the interface (so this would be host_zoom_map_observer.h,
and the class would be HostZoomMapObserver)

[Paweł Hajdan Jr., 2013/01/24 19:42:38]

Done.

+   public:
+    Observer();
+    explicit Observer(HostZoomMap* map);
+    virtual ~Observer();
+
+    // Called when the zoom level changes. |host| is a string of the hostname
+    // for which the zoom changed (empty string for temporary changes).
+    virtual void OnZoomLevelChanged(const std::string& host) = 0;
+
+   protected:
+    void Observe(HostZoomMap* map);
+
+   private:
+    friend class HostZoomMapImpl;
+
+    void HostZoomMapImplDestroyed();
+
+    HostZoomMapImpl* host_zoom_map_;
+
+    DISALLOW_COPY_AND_ASSIGN(Observer);
+  };
+
   // Copy the zoom levels from the given map. Can only be called on the UI
   // thread.
   virtual void CopyFrom(HostZoomMap* copy) = 0;