Reposition cast button on narrow videos

If a video is too narrow to display the full controls, including the cast
button, on the control bar, then use an overlay cast button instead.

BUG=423753

Review URL: https://codereview.chromium.org/658173002

git-svn-id: svn://svn.chromium.org/blink/trunk@183883 bbb929c8-8fbe-4397-9dbb-9b2b20218538