Let ChromeVox read the toast contents when it's updated.

ui/message_center/views/toast_contents_view.cc

Index: ui/message_center/views/toast_contents_view.cc
diff --git a/ui/message_center/views/toast_contents_view.cc b/ui/message_center/views/toast_contents_view.cc
index bb4b7d89d26efed5e60d9918d5963dec53167b66..b7dae986a20d9bc8adc2c99c6fd6fb3557d3c25d 100644
--- a/ui/message_center/views/toast_contents_view.cc
+++ b/ui/message_center/views/toast_contents_view.cc
@@ -84,10 +84,15 @@ views::Widget* ToastContentsView::CreateWidget(gfx::NativeView parent) {
 }
 
 void ToastContentsView::SetContents(MessageView* view) {
+  bool already_has_contents = child_count() > 0;
   RemoveAllChildViews(true);
   AddChildView(view);
   preferred_size_ = GetToastSizeForView(view);
   Layout();
+  // If it has the contents already, this invocation means an update of the
+  // popup toast, and the new contents should be read through a11y feature.
+  if (already_has_contents)
+    NotifyAccessibilityEvent(ui::AccessibilityTypes::EVENT_FOCUS, false);
 }
 
 void ToastContentsView::RevealWithAnimation(gfx::Point origin) {