Fixes possible crash in ~Display

Fixes possible crash in ~Display

If the root of a WindowTree associated with the window manager was
removed and the associated WindowTree destroyed, then Display
would still have a reference to the WindowManagerState resulting in a
possible crash.

WindowServer::DestroyTree() had code to handle this case, but it
looked up the Display using the roots of the WindowTree, so that once
the roots were removed the Display couldn't be found. Fix is to move
logic to ~WindowManagerState.

BUG=none
TEST=covered by test
R=sadrul@chromium.org

Committed: https://crrev.com/303a16654cc0cc8ef9eeacaccd122dc59cc16f83
Cr-Commit-Position: refs/heads/master@{#421196}

[sky, 2016-09-26 21:55:12]

The CQ bit was checked by sky@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-09-26 21:56:03]

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, 2016-09-26 22:53:20]

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

[commit-bot: I haz the power, 2016-09-26 22:53:21]

Dry run: Try jobs failed on following builders:
  linux_chromium_chromeos_rel_ng on master.tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[sadrul, 2016-09-27 00:56:10]

lgtm

[sky, 2016-09-27 13:13:45]

The CQ bit was checked by sky@chromium.org

[commit-bot: I haz the power, 2016-09-27 13:13:56]

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, 2016-09-27 13:56:19]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2016-09-27 13:57:45]

Description was changed from

==========
Fixes possible crash in ~Display

If the root of a WindowTree associated with the window manager was
removed and the associated WindowTree destroyed, then Display
would still have a reference to the WindowManagerState resulting in a
possible crash.

WindowServer::DestroyTree() had code to handle this case, but it
looked up the Display using the roots of the WindowTree, so that once
the roots were removed the Display couldn't be found. Fix is to move
logic to ~WindowManagerState.

BUG=none
TEST=covered by test
R=sadrul@chromium.org
==========

to

==========
Fixes possible crash in ~Display

If the root of a WindowTree associated with the window manager was
removed and the associated WindowTree destroyed, then Display
would still have a reference to the WindowManagerState resulting in a
possible crash.

WindowServer::DestroyTree() had code to handle this case, but it
looked up the Display using the roots of the WindowTree, so that once
the roots were removed the Display couldn't be found. Fix is to move
logic to ~WindowManagerState.

BUG=none
TEST=covered by test
R=sadrul@chromium.org

Committed: https://crrev.com/303a16654cc0cc8ef9eeacaccd122dc59cc16f83
Cr-Commit-Position: refs/heads/master@{#421196}
==========

[commit-bot: I haz the power, 2016-09-27 13:57:45]

Patchset 1 (id:??) landed as
https://crrev.com/303a16654cc0cc8ef9eeacaccd122dc59cc16f83
Cr-Commit-Position: refs/heads/master@{#421196}