Remove HTMLMediaElement::m_lastTimeUpdateEventMovieTime.

It was added in r41907 to work around an issue specific to the Mac's
media engine. However, it also prevented non-periodic timeupdate
events from firing (e.g., when pausing) when the underlying media
engine's time didn't change (e.g., rapidly pausing and playing).

This bug was exposed in ece894570c5f32f7f404f4c8da9ff8b9f03b75cd when
Chromium's media engine switched to stop using interpolated time values,
which means that there is a brief window of time where the media time
may remain the same.

BUG=413575

Review URL: https://codereview.chromium.org/570023004

git-svn-id: svn://svn.chromium.org/blink/trunk@182039 bbb929c8-8fbe-4397-9dbb-9b2b20218538