Add back HTMLMediaElement::m_lastTimeUpdateEventMovieTime.

I was too aggressive removing this code as it actually is nice to not
fire duplicate timeupdate events when currentTime hasn't changed.

The real fix is to leave it in, but to ignore it when firing
non-periodic timeupdate events.

For whatever it's worth, this also matches what Firefox does.

BUG=414649

Review URL: https://codereview.chromium.org/578113002

git-svn-id: svn://svn.chromium.org/blink/trunk@182260 bbb929c8-8fbe-4397-9dbb-9b2b20218538