Crash fix for window closed during Aura window capture.

Crash fix for window closed during Aura window capture.

The root cause was that while the code *did* have a WindowObserver to
stop capture when the window was destroyed, it erroneously did so by
posting a task that would be executed too late.  This change makes the
stop operation synchronous.

BUG=513103
Committed: https://crrev.com/14d3aff86097e05ccf6319114f1711b2129762e9
Cr-Commit-Position: refs/heads/master@{#347266}

[miu, 2015-09-03 20:46:43]

miu@chromium.org changed reviewers:
+ sergeyu@chromium.org

[miu, 2015-09-03 20:46:44]

sergeyu: PTAL (simple crash fix).

https://codereview.chromium.org/1327673006/diff/1/content/browser/media/captu...
File content/browser/media/capture/aura_window_capture_machine.cc (right):

https://codereview.chromium.org/1327673006/diff/1/content/browser/media/captu...
content/browser/media/capture/aura_window_capture_machine.cc:459: void
AuraWindowCaptureMachine::OnWindowDestroying(aura::Window* window) {
Note: I changed this from OnWindowDestroyed() to OnWindowDestroying() so that we
stop at the very beginning of the aura::Window destructor.  This may or may not
make a difference, but is safer because InternalStop() is making calls on the
Window's associated WindowTreeHost.

[Sergey Ulanov, 2015-09-03 22:13:05]

lgtm

[miu, 2015-09-03 22:14:38]

The CQ bit was checked by miu@chromium.org

[commit-bot: I haz the power, 2015-09-03 22:14:49]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1327673006/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1327673006/1

[commit-bot: I haz the power, 2015-09-03 22:20:41]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-03 22:21:31]

Patchset 1 (id:??) landed as
https://crrev.com/14d3aff86097e05ccf6319114f1711b2129762e9
Cr-Commit-Position: refs/heads/master@{#347266}