Fix segfault when unplugging captured external display

Fix segfault when unplugging captured external display

When unplugging an external display the WindowTreeHost is destroyed, destroying
the window and stopping the capture. However the compositor is destroyed before
the window in the WindowTreeHost destructor.

This change add an extra check that the compositor is valid before dereferencing.

BUG=441747
TEST=on link_freon unplugging external display does not result in crash when
running desktopCapture example against the external display

Committed: https://crrev.com/8a684dcb40f100ed6e02f9b0170ad4b0794a54ae
Cr-Commit-Position: refs/heads/master@{#308168}

[robert.bradford, 2014-12-12 14:15:29]

robert.bradford@intel.com changed reviewers:
+ sergeyu@chromium.org

[robert.bradford, 2014-12-12 14:15:29]

Hi Sergey, PTAL at this crasher fix. Thanks.

[Sergey Ulanov, 2014-12-12 17:42:34]

looks good, I have just couple minor nits.

https://codereview.chromium.org/801823002/diff/1/content/browser/media/captur...
File content/browser/media/capture/desktop_capture_device_aura.cc (right):

https://codereview.chromium.org/801823002/diff/1/content/browser/media/captur...
content/browser/media/capture/desktop_capture_device_aura.cc:238: // In the host
destructor the compositor is destroyed before the window
nit: add . at the end of the comment

https://codereview.chromium.org/801823002/diff/1/content/browser/media/captur...
content/browser/media/capture/desktop_capture_device_aura.cc:239: if
(desktop_window_->GetHost() && desktop_window_->GetHost()->compositor())
Store GetHost() result so it doesn't have to be called 3 times, i.e.:
 WindowTreeHost* window_host = desktop_window_->GetHost();
 if (window_host && window_host->compositor())
   window_host->compositor()->RemoveObserver(this);

[robert.bradford, 2014-12-12 18:31:39]

Hi Sergey, PTAL, i've addressed the nits you outlined. Thanks.

[Sergey Ulanov, 2014-12-12 18:46:53]

https://codereview.chromium.org/801823002/diff/20001/content/browser/media/ca...
File content/browser/media/capture/desktop_capture_device_aura.cc (right):

https://codereview.chromium.org/801823002/diff/20001/content/browser/media/ca...
content/browser/media/capture/desktop_capture_device_aura.cc:238:
aura::WindowTreeHost* wth = desktop_window_->GetHost();
Please use a more descriptive name.
From
http://google-styleguide.googlecode.com/svn/trunk/cppguide.html#General_Namin...
:
Function names, variable names, and filenames should be descriptive; eschew
abbreviation.

I suggest |window_host|.

[robert.bradford, 2014-12-12 19:05:28]

Thanks, that's good feedback. I've uploaded a changed version.

[Sergey Ulanov, 2014-12-12 19:27:54]

LGTM. Thank you for fixing it!

[robert.bradford, 2014-12-12 19:32:50]

The CQ bit was checked by robert.bradford@intel.com

[commit-bot: I haz the power, 2014-12-12 19:33:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/801823002/40001

[commit-bot: I haz the power, 2014-12-12 21:42:21]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2014-12-12 21:43:00]

Patchset 3 (id:??) landed as
https://crrev.com/8a684dcb40f100ed6e02f9b0170ad4b0794a54ae
Cr-Commit-Position: refs/heads/master@{#308168}