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..4ff11222dbf655d5b32b037766866a7ccd132b92 100644
--- a/content/public/browser/host_zoom_map.h
+++ b/content/public/browser/host_zoom_map.h
@@ -10,11 +10,13 @@
 #include <vector>
 
 #include "base/basictypes.h"
+#include "base/callback.h"
 #include "content/common/content_export.h"
 
 namespace content {
 
 class BrowserContext;
+class HostZoomMapImpl;

[jam, 2013/01/30 17:50:44]

nit: not needed anymore

 class ResourceContext;
 
 // Maps hostnames to custom zoom levels.  Written on the UI thread and read on
@@ -22,6 +24,8 @@ class ResourceContext;
 // thread, and it'll delete itself on the UI thread as well.
 class HostZoomMap {
  public:
+  typedef base::Callback<void(const std::string&)> ZoomLevelChangedCallback;

[jam, 2013/01/30 17:50:44]

nit: the convention in the content api is to declare the callbacks right above
the method that uses them

please document what the string parameter in the callback is used for

+
   CONTENT_EXPORT static HostZoomMap* GetForBrowserContext(
       BrowserContext* browser_context);
 
@@ -50,6 +54,12 @@ class HostZoomMap {
   virtual double GetDefaultZoomLevel() const = 0;
   virtual void SetDefaultZoomLevel(double level) = 0;;
 
+  // Add and remove zoom level changed callbacks.
+  virtual void AddZoomLevelChangedCallback(
+      ZoomLevelChangedCallback callback) = 0;

[jam, 2013/01/30 17:50:44]

nit: callbacks, like other complex types, are passed as const references

+  virtual void RemoveZoomLevelChangedCallback(
+      ZoomLevelChangedCallback callback) = 0;
+
  protected:
   virtual ~HostZoomMap() {}
 };