Remove incorrect ASSERT from HTMLMediaElement::hasPendingElement

R=dpranke@chromium.org,kbr@chromium.org,philipj@opera.com
BUG=400659

Review URL: https://codereview.chromium.org/584633004

git-svn-id: svn://svn.chromium.org/blink/trunk@182278 bbb929c8-8fbe-4397-9dbb-9b2b20218538