Reland "Only alert accessibility when a notification's text changes"

This reverts commit ad618241.

Reason for revert: <INSERT REASONING HERE>
Chromeos-dbg bots are now green (for chromevox); this change was unlikely to be at fault.

Original change's description:
> Revert "Only alert accessibility when a notification's text changes"
> 
> This reverts commit feb758c3.
> 
> Reason for revert: <INSERT REASONING HERE>
> SpokenFeedbackTests failing on chromeos dbg
> Original change's description:
> > Only alert accessibility when a notification's text changes
> > 
> > Bug: 867426
> > Change-Id: I7f220566f3e7a2b4ce7ef639d012f4323b762208
> > Reviewed-on: https://chromium-review.googlesource.com/1166231
> > Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
> > Commit-Queue: David Tseng <dtseng@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#581474}
> 
> TBR=stevenjb@chromium.org,dtseng@chromium.org
> 
> Change-Id: I8b23c03e08642caa7b05d2ce88e6f330dd056c37
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: 867426
> Reviewed-on: https://chromium-review.googlesource.com/1167603
> Reviewed-by: David Tseng <dtseng@chromium.org>
> Commit-Queue: David Tseng <dtseng@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#581607}

TBR=stevenjb@chromium.org,dtseng@chromium.org

Change-Id: I0b1270e0ebdbfbca89baefe9a9f676062862479e
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 867426
Reviewed-on: https://chromium-review.googlesource.com/1168329Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581875}

ui/message_center/views/message_popup_view.cc

@@ -67,11 +67,17 @@ MessagePopupView::~MessagePopupView() {
 }
 
 void MessagePopupView::UpdateContents(const Notification& notification) {
+  ui::AXNodeData old_data;
+  message_view_->GetAccessibleNodeData(&old_data);
   message_view_->UpdateWithNotification(notification);
   popup_collection_->NotifyPopupResized();
   if (notification.rich_notification_data()
           .should_make_spoken_feedback_for_popup_updates) {
-    NotifyAccessibilityEvent(ax::mojom::Event::kAlert, true);
+    ui::AXNodeData new_data;
+    message_view_->GetAccessibleNodeData(&new_data);
+    if (old_data.GetStringAttribute(ax::mojom::StringAttribute::kName) !=
+        new_data.GetStringAttribute(ax::mojom::StringAttribute::kName))
+      NotifyAccessibilityEvent(ax::mojom::Event::kAlert, true);
   }
 }