Add a mediaControlsOverlayPlayButtonEnabled setting

This will be followed by a patch to enable the setting on Android and
then a patch to actually let the setting enable/disable the button.

BUG=366675

Review URL: https://codereview.chromium.org/250533003

git-svn-id: svn://svn.chromium.org/blink/trunk@172737 bbb929c8-8fbe-4397-9dbb-9b2b20218538