Do not fire an initial periodic timeupdate event while currentTime is 0

m_lastTimeUpdateEventMovieTime has been initialized to infinity for as
long as the code has existed: http://trac.webkit.org/changeset/41907

This may be the cause of a spurious timeupdate event on Mac 10.9 in a
recently added test: https://codereview.chromium.org/578243003

TEST=media/gc-while-playing.html

BUG=400659

Review URL: https://codereview.chromium.org/587373002

git-svn-id: svn://svn.chromium.org/blink/trunk@182442 bbb929c8-8fbe-4397-9dbb-9b2b20218538