Null out context_factory in window_tree_host on shutdown

Null out context_factory in window_tree_host on shutdown

A context factory is specific to a particular native viewport so we
shouldn't leak it between different WindowTreeHostImpl instances. This
fixes some crashes in mojo_view_manager_unittests.

R=sky@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=291196

[sky, 2014-08-20 21:37:54]

LGTM

[jamesr, 2014-08-20 21:40:08]

The CQ bit was checked by jamesr@chromium.org

[commit-bot: I haz the power, 2014-08-20 21:40:42]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/jamesr@chromium.org/494883002/1

[jamesr, 2014-08-20 21:41:58]

The CQ bit was checked by jamesr@chromium.org

[commit-bot: I haz the power, 2014-08-20 21:42:25]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/jamesr@chromium.org/494883002/20001

[commit-bot: I haz the power, 2014-08-20 23:07:17]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  linux_gpu on tryserver.chromium.gpu
(http://build.chromium.org/p/tryserver.chromium.gpu/builders/linux_gpu/builds/...)
  android_clang_dbg on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/android_clang_d...)
  linux_chromium_rel_swarming on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[commit-bot: I haz the power, 2014-08-20 23:12:05]

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

[commit-bot: I haz the power, 2014-08-20 23:12:06]

Try jobs failed on following builders:
  linux_gpu on tryserver.chromium.gpu
(http://build.chromium.org/p/tryserver.chromium.gpu/builders/linux_gpu/builds/...)
  android_clang_dbg on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/android_clang_d...)

[jamesr, 2014-08-21 20:27:56]

The CQ bit was checked by jamesr@chromium.org

[commit-bot: I haz the power, 2014-08-21 20:31:40]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/jamesr@chromium.org/494883002/20001

[commit-bot: I haz the power, 2014-08-21 21:34:55]

Committed patchset #2 (20001) as 291196