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

http://www.whatwg.org/specs/web-apps/current-work/multipage/the-video-element.html#user-interface

Added to the spec in http://html5.org/r/8315 after a long discussion:
http://lists.whatwg.org/htdig.cgi/whatwg-whatwg.org/2013-November/041662.html

Firefox already implements click to play/pause when controls are
visible, but that implementation predates the spec so accesskey does not
work.

HTMLMediaElement::togglePlayState() is made public so that it can be
used to implement play/pause in MediaControls also.

The video-click-dblckick-standalone.html test has been expected to fail
since April 2013, r148246, so remove it now since the the results would
change. The new tests provide sufficient coverage.

BUG=354746
TEST=LayoutTests/media/activation-behavior.html
     LayoutTests/media/activation-behavior-accesskey.html
     LayoutTests/media/activation-behavior-shadow.html

Review URL: https://codereview.chromium.org/208483002

git-svn-id: svn://svn.chromium.org/blink/trunk@169812 bbb929c8-8fbe-4397-9dbb-9b2b20218538