Bug 1535005 - part1 : no need to update 'mLastTimeMarchesOnCalled' in 'DidSeek()' r=jya

According to spec [1],  `mLastTimeMarchesOnCalled` is used to represent the `last time` in step3. It's used to record last time we run `TimeMarchOn()`, so there is no need to upate it on `DidSeek()`.

[1] https://html.spec.whatwg.org/multipage/media.html#time-marches-on

Differential Revision: https://phabricator.services.mozilla.com/D29094

dom/html/TextTrackManager.cpp

@@ -232,12 +232,6 @@ void TextTrackManager::RemoveTextTrack(TextTrack* aTextTrack,
 
 void TextTrackManager::DidSeek() {
   WEBVTT_LOG("DidSeek");
-  if (mMediaElement) {
-    mLastTimeMarchesOnCalled =
-        media::TimeUnit::FromSeconds(mMediaElement->CurrentTime());
-    WEBVTT_LOGV("DidSeek set mLastTimeMarchesOnCalled %lf",
-                mLastTimeMarchesOnCalled.ToSeconds());
-  }
   mHasSeeked = true;
 }