Don't show media controls on focus if 'controls' is not set.

Previously when an HTMLMediaElement received focus, the media controls
would be shown regardless of whether 'controls' was set.
It's possible for an HTMLMediaElement to receive focus despite not having
'controls' set, if it has a 'tabindex'.

BUG=424625

Review URL: https://codereview.chromium.org/692643002

git-svn-id: svn://svn.chromium.org/blink/trunk@184803 bbb929c8-8fbe-4397-9dbb-9b2b20218538