Simplify HTMLTrackElement::didCompleteLoad

It doesn't need to be virtual and didn't use the first argument.

BUG=313287

Review URL: https://codereview.chromium.org/51883010

git-svn-id: svn://svn.chromium.org/blink/trunk@160971 bbb929c8-8fbe-4397-9dbb-9b2b20218538