tabbed_pane: notify a11y event on focus

In MdTab::OnFocus, it does not notify a11y event of focus, which causes
the ChromeVox focus out of sync. This patch notifies the event as we do
in the Tab::OnFocus.

Bug: 876388
Test: manual
Change-Id: I332c3dfa355d9ab41cf5848f549cde164d73b045
Reviewed-on: https://chromium-review.googlesource.com/c/1406196Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622062}

ui/views/controls/tabbed_pane/tabbed_pane.cc

@@ -383,6 +383,13 @@ void MdTab::OnFocus() {
                         ui::NativeTheme::kColorId_FocusedBorderColor),
                     0x66)));
   }
+
+  // When the tab gains focus, send an accessibility event indicating that the
+  // contents are focused. When the tab loses focus, whichever new View ends up
+  // with focus will send an ax::mojom::Event::kFocus of its own, so there's no
+  // need to send one in OnBlur().
+  if (contents())
+    contents()->NotifyAccessibilityEvent(ax::mojom::Event::kFocus, true);
   SchedulePaint();
 }