aura: Reland r266681 to debug WindowObserver crashes.

aura: Reland r266681 to debug WindowObserver crashes.

Add code to make the aura::WindowObserver interface crash
when it's destroyed without having unregistered its interest
in all aura::Windows that it was observing. This will
hopefully help track down the source of crashes that are
seen when closing video windows on Google Play.

https://codereview.chromium.org/303163003/ hopefully fixes
the test crashes that resulted in this change being reverted
earlier.

BUG=365364
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=274086

[Daniel Erat, 2014-05-29 23:46:26]

This is dependent on https://codereview.chromium.org/303163003/ going in first.

[flackr, 2014-05-30 00:12:22]

lgtm

https://codereview.chromium.org/309573004/diff/1/ui/aura/window_observer.cc
File ui/aura/window_observer.cc (right):

https://codereview.chromium.org/309573004/diff/1/ui/aura/window_observer.cc#n...
ui/aura/window_observer.cc:1: // Copyright (c) 2013 The Chromium Authors. All
rights reserved.
nit: This copyright notice is out of date now :-).

[sky, 2014-05-30 02:34:34]

LGTM

[Daniel Erat, 2014-05-31 04:24:56]

The CQ bit was checked by derat@chromium.org

[commit-bot: I haz the power, 2014-05-31 04:28:40]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/derat@chromium.org/309573004/1

[commit-bot: I haz the power, 2014-05-31 22:55:55]

Change committed as 274086