Push cue timeline management out of LoadableTextTrack

Push cue timeline management out of LoadableTextTrack

When a LoadableTextTrack receives cues from the loader, it adds them to
the cue list and notifies the CueTimeline. This is the only place where
this kind of TextTrack state is directly handled by subclasses of
TextTrack. Moving this to TextTrack we can "lock down" state - cue list
and timeline - some more, and have updates due to list-mutations in a
single place (there's currently still updates in HTMLMediaElement though.)
This also makes LoadableTextTrack look more like a provider of cues.
Transport of cues needs to be updated to use a wider type. Also make use
of Vector::swap in VTTParser while changing the signatures.

BUG=321654
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=190657

[fs, 2015-02-20 16:33:48]

fs@opera.com changed reviewers:
+ philipj@opera.com

[philipj_slow, 2015-02-23 13:47:20]

The CQ bit was checked by philipj@opera.com

[philipj_slow, 2015-02-23 13:47:20]

lgtm

[commit-bot: I haz the power, 2015-02-23 13:48:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/941973005/1

[commit-bot: I haz the power, 2015-02-23 13:52:13]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=190657