Don't fire a timeupdate event if the seek time hasn't changed.

This fixes the last bug that caused timeupdate events to fire before
they should have. As a result, fix a layout test that accidentally
removed its timeupdate event listener prior to when it should have.

BUG=223476

Review URL: https://codereview.chromium.org/346003006

git-svn-id: svn://svn.chromium.org/blink/trunk@176786 bbb929c8-8fbe-4397-9dbb-9b2b20218538