Support negative timestamps of TextTrackCue

Ensure proper behaviour for negative timestamps of TextTrackCue.
1. Cues with negative startTime should become active from 0s.
2. Cues with negative startTime and endTime should never be active.

Bug: 314032
Change-Id: Ib53710e58be0be770c933ea8c3c4709a0e5dec0d
Reviewed-on: https://chromium-review.googlesource.com/863270
Commit-Queue: srirama chandra sekhar <srirama.m@samsung.com>
Reviewed-by: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#529012}
void TextTrackCueList::Trace(blink::Visitor* visitor) {
  visitor->Trace(list_);
  ScriptWrappable::Trace(visitor);
}

void TextTrackCueList::Trace(blink::Visitor* visitor) {
  visitor->Trace(list_);
  ScriptWrappable::Trace(visitor);
}
