[WIP] Use the flat tree for hover handling.

[WIP] Use the flat tree for hover handling.

Modulo spec debate like[1], this is much simpler and closer to the spec than
what it was implemented.

This doesn't actually fix issue 705984, but it's a required cleanup, and I've
verified that the display: contents node is added to the :hover chain (if
there's an intermediate <div>), so I probably need to investigate how hit testing
and style invalidation is done, etc, to track down that bug.

Still this is worth landing IMO.

BUG=

[emilio, 2017-03-31 10:13:00]

The CQ bit was checked by ecobos@igalia.com to run a CQ dry run

[commit-bot: I haz the power, 2017-03-31 10:13:11]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-03-31 11:44:15]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2017-03-31 11:44:16]

Dry run: Try jobs failed on following builders:
  win_chromium_rel_ng on master.tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_rel_...)