Fire native accessibility events when changing a views combobox.

The second argument to NotifyAccessibilityEvent is whether we should
fire a native event, otherwise just a Chrome OS event is fired.
This should almost always be true. Changing this one case from false
to true fixes the linked bug where the combobox in the bookmarks
popup didn't announce when changing.

Bug: 779302
Change-Id: Ida63037d8a7a01e43150e7e3e2398a965d8405eb
Reviewed-on: https://chromium-review.googlesource.com/809728Reviewed-by: Trent Apted <tapted@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521998}

ui/views/controls/combobox/combobox.cc

@@ -996,7 +996,7 @@ void Combobox::OnMenuClosed(Button::ButtonState original_button_state) {
 }
 
 void Combobox::OnPerformAction() {
-  NotifyAccessibilityEvent(ui::AX_EVENT_VALUE_CHANGED, false);
+  NotifyAccessibilityEvent(ui::AX_EVENT_VALUE_CHANGED, true);
   SchedulePaint();
 
   // This combobox may be deleted by the listener.