Set focus to the "Switch to..." button on showing the overlay.

By default, focus is on the BODY element. This is not useful, especially for
accessibility.

BUG=316718
TEST=added to browser_test

Review URL: https://codereview.chromium.org/82553009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236796 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236796