If the media controls are visible they should always grab clicks

Once we have decided to show the media controls it makes no sense to
allow the page to change their handling. This CL ensures that, if the
media controls are shown, then clicks on them actually get to them.

BUG=388738
BUG=269454

Review URL: https://codereview.chromium.org/406213002

git-svn-id: svn://svn.chromium.org/blink/trunk@179397 bbb929c8-8fbe-4397-9dbb-9b2b20218538