Revert of [headless] Use individual aura::WindowTreeHosts per WebContents.

Revert of [headless] Use individual aura::WindowTreeHosts per WebContents. (patchset #1 id:1 of https://codereview.chromium.org/2787373002/ )

Reason for revert:
Suspect this is causing failures of TargetDomainCreateTwoContexts.RunAsyncTest in browser_side_navigation_headless_browsertests.

https://build.chromium.org/p/chromium.linux/builders/Linux%20Tests%20%28dbg%29%281%29?numbuilds=200

Original issue's description:
> [headless] Use individual aura::WindowTreeHosts per WebContents.
> 
> This way, compositing concerns are handled separately per WebContents,
> which is required for BeginFrameControl.
> 
> BUG=646774
> 
> Review-Url: https://codereview.chromium.org/2787373002
> Cr-Commit-Position: refs/heads/master@{#461200}
> Committed: https://chromium.googlesource.com/chromium/src/+/56b635d784ad0eacbd06e744e43c115e48993a04

TBR=skyostil@chromium.org,eseckler@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=646774
Review-Url: https://codereview.chromium.org/2792883002
Cr-Commit-Position: refs/heads/master@{#461367}
Committed: https://chromium.googlesource.com/chromium/src/+/9f50c75ebf30e61ad79311c1271655c7573261c5

[Timothy Loh, 2017-04-03 01:50:45]

The CQ bit was checked by timloh@chromium.org

[Timothy Loh, 2017-04-03 01:50:45]

Created Revert of [headless] Use individual aura::WindowTreeHosts per
WebContents.

[commit-bot: I haz the power, 2017-04-03 01:50:52]

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, 2017-04-03 03:57:10]

CQ is committing da patch.

Bot data: {"patchset_id": 1, "attempt_start_ts": 1491184245432780, "parent_rev":
"b06db87db43c1d78a7e23b87383fd61e011bb8b6", "commit_rev":
"9f50c75ebf30e61ad79311c1271655c7573261c5"}

[commit-bot: I haz the power, 2017-04-03 03:57:52]

Description was changed from

==========
Revert of [headless] Use individual aura::WindowTreeHosts per WebContents.
(patchset #1 id:1 of https://codereview.chromium.org/2787373002/ )

Reason for revert:
Suspect this is causing failures of TargetDomainCreateTwoContexts.RunAsyncTest
in browser_side_navigation_headless_browsertests.

https://build.chromium.org/p/chromium.linux/builders/Linux%20Tests%20%28dbg%2...

Original issue's description:
> [headless] Use individual aura::WindowTreeHosts per WebContents.
> 
> This way, compositing concerns are handled separately per WebContents,
> which is required for BeginFrameControl.
> 
> BUG=646774
> 
> Review-Url: https://codereview.chromium.org/2787373002
> Cr-Commit-Position: refs/heads/master@{#461200}
> Committed:
https://chromium.googlesource.com/chromium/src/+/56b635d784ad0eacbd06e744e43c...

TBR=skyostil@chromium.org,eseckler@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=646774
==========

to

==========
Revert of [headless] Use individual aura::WindowTreeHosts per WebContents.
(patchset #1 id:1 of https://codereview.chromium.org/2787373002/ )

Reason for revert:
Suspect this is causing failures of TargetDomainCreateTwoContexts.RunAsyncTest
in browser_side_navigation_headless_browsertests.

https://build.chromium.org/p/chromium.linux/builders/Linux%20Tests%20%28dbg%2...

Original issue's description:
> [headless] Use individual aura::WindowTreeHosts per WebContents.
> 
> This way, compositing concerns are handled separately per WebContents,
> which is required for BeginFrameControl.
> 
> BUG=646774
> 
> Review-Url: https://codereview.chromium.org/2787373002
> Cr-Commit-Position: refs/heads/master@{#461200}
> Committed:
https://chromium.googlesource.com/chromium/src/+/56b635d784ad0eacbd06e744e43c...

TBR=skyostil@chromium.org,eseckler@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=646774

Review-Url: https://codereview.chromium.org/2792883002
Cr-Commit-Position: refs/heads/master@{#461367}
Committed:
https://chromium.googlesource.com/chromium/src/+/9f50c75ebf30e61ad79311c12716...
==========

[commit-bot: I haz the power, 2017-04-03 03:57:53]

Committed patchset #1 (id:1) as
https://chromium.googlesource.com/chromium/src/+/9f50c75ebf30e61ad79311c12716...

[Eric Seckler, 2017-04-03 09:35:20]

lgtm. seems like this was caused by timing changes that surfaced a race in the
test. fix for the test is out on https://codereview.chromium.org/2794883002/.