Add ripple effect to action in expand button.

ui/message_center/views/notification_view_md.cc

Index: ui/message_center/views/notification_view_md.cc
diff --git a/ui/message_center/views/notification_view_md.cc b/ui/message_center/views/notification_view_md.cc
index 9625f54d48f1e5f11fa31e8990e3ee3dcae9ae9e..551a8b0de64d2aa241af01214c07e35f99070649 100644
--- a/ui/message_center/views/notification_view_md.cc
+++ b/ui/message_center/views/notification_view_md.cc
@@ -434,8 +434,7 @@ void NotificationViewMD::ButtonPressed(views::Button* sender,
 
   // Tapping anywhere on |header_row_| can expand the notification, though only
   // |expand_button| can be focused by TAB.
-  if (IsExpandable() &&
-      (sender == header_row_ || sender == header_row_->expand_button())) {
+  if (IsExpandable() && sender == header_row_) {
     ToggleExpanded();
     Layout();
     SchedulePaint();