Allow media control buttons to be returned for accessibility

For elements in the shadow tree, the hit test only returns the shadow host.
For media controls, the shadow host is the media element.
As a result, accessibility won't work on media control buttons.
This change makes it possible to return media controls during hit test.

BUG=363821

Review URL: https://codereview.chromium.org/683473004

git-svn-id: svn://svn.chromium.org/blink/trunk@184484 bbb929c8-8fbe-4397-9dbb-9b2b20218538