Clean up around text track display state updates

Clean up around text track display state updates

Several minor oddities are changed to be hopefully less odd:

 * CueTimeline need not check activeSetChanged due to an earlier
   |if (!activeSetChanged) return|.
 * Move one-time setup of m_displayTree and m_cueBackgroundBox to where
   they are created.
 * ASSERT the preconditions guaranteed in VTTCue::updateDisplay() and
   getDisplayTree(), guaranteed by TextTrackContainer::updateDisplay.
 * Never remove and m_cueBackgroundBox from m_displayTree, just leave it
   there and assert that it is so.
 * Document where m_cueBackgroundBox is actually populated.
 * Remove a bunch of spec comments where the code doesn't resemble the
   shape of the spec and FIXMEs that aren't anywhere close to where they
   would actually be fixed. A link to http://wkb.ug/79916 remains in
   VTTCueBox::applyCSSProperties().

BUG=322434
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=192113

[philipj_slow, 2015-03-18 16:53:55]

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

[philipj_slow, 2015-03-18 16:53:55]

PTAL. Random things that I found while trying to take the crazy out of
updateDisplayTree and want to land first to make that CL cleaner.

[fs, 2015-03-18 17:02:05]

The CQ bit was checked by fs@opera.com

[fs, 2015-03-18 17:02:06]

lgtm

[commit-bot: I haz the power, 2015-03-18 17:02:26]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1022523002/1

[commit-bot: I haz the power, 2015-03-18 18:39:19]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=192113