MacViews: Reveal scrollbars when resting on the trackpad.

ui/views/controls/scroll_view.cc

Index: ui/views/controls/scroll_view.cc
diff --git a/ui/views/controls/scroll_view.cc b/ui/views/controls/scroll_view.cc
index b62972d894f5f99ea6e2fd30ba37486025bf7bb6..87e1414d86e4320b14836ef67c52dd8f36e3a185 100644
--- a/ui/views/controls/scroll_view.cc
+++ b/ui/views/controls/scroll_view.cc
@@ -472,9 +472,20 @@ void ScrollView::OnMouseExited(const ui::MouseEvent& event) {
 
 void ScrollView::OnScrollEvent(ui::ScrollEvent* event) {
 #if defined(OS_MACOSX)
+  if (!contents_)
+    return;
+
   // TODO(tapted): Send |event| to a cc::InputHandler. For now, there's nothing
   // to do because Widget::OnScrollEvent() will automatically process an
   // unhandled ScrollEvent as a MouseWheelEvent.
+
+  // A direction might not be known when the event stream starts, notify both
+  // scrollbars that they may be about scroll, or that they may need to cancel
+  // UI feedback once the scrolling direction is known.
+  if (horiz_sb_)
+    horiz_sb_->ObserveScrollEvent(*event);
+  if (vert_sb_)
+    vert_sb_->ObserveScrollEvent(*event);
 #endif
 }