Call RemoveObserver from OnWidgetDestroying

Call RemoveObserver from OnWidgetDestroying

BUG=487693
Committed: https://crrev.com/35337bb61d6e50f251e547c0ee2e7a1f00d8f1e1
Cr-Commit-Position: refs/heads/master@{#342734}

[dmazzoni, 2015-08-10 15:57:32]

dmazzoni@chromium.org changed reviewers:
+ sadrul@chromium.org

[sadrul, 2015-08-10 16:50:23]

Would it be possible to add a test for this?

[dmazzoni, 2015-08-10 21:28:13]

Done. I wasn't able to come up with an easy way to trigger
NativeViewAccessibility being deleted between OnWidgetDestroying and
OnWidgetDestroyed, but I was able to trigger the crash by explicitly deleting it
from within OnWidgetDestroying in a subclass used by a test.

Without the fix, this test causes ASAN to fail with a similar stack trace, and
with the fix it's clean.

[sadrul, 2015-08-10 21:30:00]

Cool. Thanks! lgtm

[dmazzoni, 2015-08-10 21:31:14]

The CQ bit was checked by dmazzoni@chromium.org

[commit-bot: I haz the power, 2015-08-10 21:32:21]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1287463002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1287463002/20001

[commit-bot: I haz the power, 2015-08-10 23:33:47]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-08-10 23:34:39]

Patchset 2 (id:??) landed as
https://crrev.com/35337bb61d6e50f251e547c0ee2e7a1f00d8f1e1
Cr-Commit-Position: refs/heads/master@{#342734}