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/1406196
Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622062}
1 file changed