Disable the activation behavior of media elements (click to play/pause)

Sites that implement click to play/pause using JavaScript without
calling preventDefault() have unfortunately surfaced. This is serious
since at worst the videos are completely unplayable.

A remedy would be http://crbug.com/269454 so that no events are fired
when the native controls are used. Revert the activation behavior for
now, in order to try again after that has been fixed.

This partially reverts r169812 and r170789.

BUG=354746, 358675

Review URL: https://codereview.chromium.org/249483002

git-svn-id: svn://svn.chromium.org/blink/trunk@172484 bbb929c8-8fbe-4397-9dbb-9b2b20218538