Reset media controls when load() is called

Calling reset() in configureMediaControls() is a slightly bigger hammer
than should be needed, but it isn't called very often so it shouldn't
matter in practice. Ideally MediaControls should be hooked to the same
fine-grained events as scripts see, but that's for the future.

BUG=350310

Review URL: https://codereview.chromium.org/190333002

git-svn-id: svn://svn.chromium.org/blink/trunk@168776 bbb929c8-8fbe-4397-9dbb-9b2b20218538