content: convert zoom notifications to observer usage

content/browser/host_zoom_map_impl.cc

Index: content/browser/host_zoom_map_impl.cc
diff --git a/content/browser/host_zoom_map_impl.cc b/content/browser/host_zoom_map_impl.cc
index 942d6f49c61064c6e06e4d6ff73c30e77ad77360..aa5a870434a640a2f9d150c05c320f95b7f690ac 100644
--- a/content/browser/host_zoom_map_impl.cc
+++ b/content/browser/host_zoom_map_impl.cc
@@ -90,10 +90,8 @@ void HostZoomMapImpl::SetZoomLevel(const std::string& host, double level) {
     }
   }
 
-  NotificationService::current()->Notify(
-      NOTIFICATION_ZOOM_LEVEL_CHANGED,
-      Source<HostZoomMap>(this),
-      Details<const std::string>(&host));
+  for (size_t i = 0; i < zoom_level_changed_callbacks_.size(); i++)
+    zoom_level_changed_callbacks_[i].Run(host);
 }
 
 double HostZoomMapImpl::GetDefaultZoomLevel() const {
@@ -104,6 +102,22 @@ void HostZoomMapImpl::SetDefaultZoomLevel(double level) {
   default_zoom_level_ = level;
 }
 
+void HostZoomMapImpl::AddZoomLevelChangedCallback(
+    const ZoomLevelChangedCallback& callback) {
+  zoom_level_changed_callbacks_.push_back(callback);
+}
+
+void HostZoomMapImpl::RemoveZoomLevelChangedCallback(
+    const ZoomLevelChangedCallback& callback) {
+  for (size_t i = 0; i < zoom_level_changed_callbacks_.size(); i++) {
+    if (zoom_level_changed_callbacks_[i].Equals(callback)) {
+      zoom_level_changed_callbacks_.erase(
+          zoom_level_changed_callbacks_.begin() + i);
+      return;
+    }
+  }
+}
+
 double HostZoomMapImpl::GetTemporaryZoomLevel(int render_process_id,
                                               int render_view_id) const {
   base::AutoLock auto_lock(lock_);
@@ -145,11 +159,8 @@ void HostZoomMapImpl::SetTemporaryZoomLevel(int render_process_id,
     }
   }
 
-  std::string host;
-  NotificationService::current()->Notify(
-      NOTIFICATION_ZOOM_LEVEL_CHANGED,
-      Source<HostZoomMap>(this),
-      Details<const std::string>(&host));
+  for (size_t i = 0; i < zoom_level_changed_callbacks_.size(); i++)
+    zoom_level_changed_callbacks_[i].Run(std::string());
 }
 
 void HostZoomMapImpl::Observe(int type,