Move creation and ownership of HostZoomMap to content instead of having every embedder do this.

content/browser/host_zoom_map_impl.h

Index: content/browser/host_zoom_map_impl.h
===================================================================
--- content/browser/host_zoom_map_impl.h	(revision 122827)
+++ content/browser/host_zoom_map_impl.h	(working copy)
@@ -12,6 +12,7 @@
 
 #include "base/compiler_specific.h"
 #include "base/message_loop_helpers.h"
+#include "base/supports_user_data.h"
 #include "base/synchronization/lock.h"
 #include "content/public/browser/host_zoom_map.h"
 #include "content/public/browser/notification_observer.h"
@@ -21,7 +22,8 @@
 // to notifications on there (and holds a NotificationRegistrar).
 class CONTENT_EXPORT HostZoomMapImpl
     : public NON_EXPORTED_BASE(content::HostZoomMap),
-      public content::NotificationObserver {
+      public content::NotificationObserver,
+      public base::SupportsUserData::Data {
  public:
   HostZoomMapImpl();