Suppress more media timeupdate events when seeking

Playback progress timer firing should not cause timeupdate event
scheduling while seeking, and neither should active text track cue
updating cause timeupdate event scheduling while seeking.
Timeupdate event is scheduled during finishSeek for these scenarios.

BUG=261722
R=acolwell@chromium.org,adamk@chromium.org
TEST=no media layout test regression on linux, video-seek-to-[duration,middle] pass on Chrome for Android

Review URL: https://chromiumcodereview.appspot.com/19842005

git-svn-id: svn://svn.chromium.org/blink/trunk@154667 bbb929c8-8fbe-4397-9dbb-9b2b20218538