Make OnMessageCenterScrolled fire by fling.

OnMessageCenterScrolled should be fired when it's scrolled by fling.
Otherwise, stacking notification counter https://crrev.com/c/1168945 and
notification corners https://crrev.com/c/1161710 might show invalid
state with touch screen.

TEST=manual
BUG=none

Change-Id: I0a647bdd5af979775ef4ef4e8c470a96cbfd299a
Reviewed-on: https://chromium-review.googlesource.com/1172233Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582863}

ash/message_center/message_center_scroll_bar.cc

@@ -68,4 +68,11 @@ void MessageCenterScrollBar::OnGestureEvent(ui::GestureEvent* event) {
     observer_->OnMessageCenterScrolled();
 }
 
+bool MessageCenterScrollBar::OnScroll(float dx, float dy) {
+  bool result = views::OverlayScrollBar::OnScroll(dx, dy);
+  if (observer_)
+    observer_->OnMessageCenterScrolled();
+  return result;
+}
+
 }  // namespace ash

ash/message_center/message_center_scroll_bar.h

@@ -33,6 +33,9 @@ class MessageCenterScrollBar : public views::OverlayScrollBar {
   // ui::EventHandler overrides:
   void OnGestureEvent(ui::GestureEvent* event) override;
 
+  // views::ScrollDelegate overrides:
+  bool OnScroll(float dx, float dy) override;
+
   // False if no event is recorded yet. True if the first event is recorded.
   bool stats_recorded_ = false;