Indent comments in HTMLMediaElement.h

Drop the "DOM API" comment because not everything below that point
is part of the DOM API anyway.

BUG=none
NOTRY=true

Review URL: https://codereview.chromium.org/54673003

git-svn-id: svn://svn.chromium.org/blink/trunk@161062 bbb929c8-8fbe-4397-9dbb-9b2b20218538