Remove HTMLMediaElement::isPlaying()

It's unused as of r182247:
https://codereview.chromium.org/552303006

BUG=400659

Review URL: https://codereview.chromium.org/583203002

git-svn-id: svn://svn.chromium.org/blink/trunk@182333 bbb929c8-8fbe-4397-9dbb-9b2b20218538