Fix zoom level alert for screen readers

NVDA doesn't seem to be picking up the accessibility event on the zoom
level label. This change sends an alert event on the zoom bubble
instead.

Bug: 849472
Change-Id: I391e01bb952325b133cf9362d80c473140717a44
Reviewed-on: https://chromium-review.googlesource.com/c/1305242Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Commit-Queue: Collin Baker <collinbaker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604800}

chrome/browser/ui/views/location_bar/zoom_bubble_view.cc

@@ -273,6 +273,8 @@ ZoomBubbleView* ZoomBubbleView::GetZoomBubble() {
 
 void ZoomBubbleView::Refresh() {
   UpdateZoomPercent();
+  GetWidget()->GetRootView()->NotifyAccessibilityEvent(ax::mojom::Event::kAlert,
+                                                       true);
   StartTimerIfNecessary();
 }
 
@@ -569,7 +571,6 @@ void ZoomBubbleView::SetExtensionInfo(const extensions::Extension* extension) {
 void ZoomBubbleView::UpdateZoomPercent() {
   label_->SetText(base::FormatPercent(
       zoom::ZoomController::FromWebContents(web_contents())->GetZoomPercent()));
-  label_->NotifyAccessibilityEvent(ax::mojom::Event::kTextChanged, true);
 }
 
 void ZoomBubbleView::UpdateZoomIconVisibility() {