content: convert zoom notifications to observer usage

chrome/browser/ui/gtk/browser_toolbar_gtk.h

Index: chrome/browser/ui/gtk/browser_toolbar_gtk.h
diff --git a/chrome/browser/ui/gtk/browser_toolbar_gtk.h b/chrome/browser/ui/gtk/browser_toolbar_gtk.h
index 932d142815695ada8b7ff483f3135d808636dc0e..e988156ba1836dd0af6233738abf4611eaba833e 100644
--- a/chrome/browser/ui/gtk/browser_toolbar_gtk.h
+++ b/chrome/browser/ui/gtk/browser_toolbar_gtk.h
@@ -15,6 +15,7 @@
 #include "chrome/browser/ui/gtk/custom_button.h"
 #include "chrome/browser/ui/gtk/menu_gtk.h"
 #include "chrome/browser/ui/toolbar/wrench_menu_model.h"
+#include "content/public/browser/host_zoom_map.h"
 #include "content/public/browser/notification_observer.h"
 #include "content/public/browser/notification_registrar.h"
 #include "ui/base/accelerators/accelerator.h"
@@ -112,6 +113,8 @@ class BrowserToolbarGtk : public CommandObserver,
                          bool should_restore_state);
 
  private:
+  void OnZoomLevelChanged(const std::string& host);
+
   // Connect/Disconnect signals for dragging a url onto the home button.
   void SetUpDragForHomeButton();
 
@@ -211,6 +214,7 @@ class BrowserToolbarGtk : public CommandObserver,
   StringPrefMember home_page_;
   BooleanPrefMember home_page_is_new_tab_page_;
 
+  content::HostZoomMap::ZoomLevelChangedCallback zoom_callback_;
   content::NotificationRegistrar registrar_;
 
   // A GtkEntry that isn't part of the hierarchy. We keep this for native