Add checks to verify that all observed_windows_ are targeted in overview windows_.

Add checks to verify that all observed_windows_ are targeted in overview windows_.

BUG=322119
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=236976

[flackr, 2013-11-22 22:39:42]

I'm at a loss for what conditions could be causing the three crash reports I've
found which seem to be calling RemoveObserver on a window which no longer
exists. As far as I can tell these added checks should never fire but given the
crashes in WindowSelector::~WindowSelector one of these checks should fire
first. This should just cause it to crash earlier and hopefully point at the
problem. PTAL, thanks!

[flackr, 2013-11-22 22:42:34]

https://codereview.chromium.org/83363004/diff/1/ash/wm/overview/window_select...
File ash/wm/overview/window_selector.cc (right):

https://codereview.chromium.org/83363004/diff/1/ash/wm/overview/window_select...
ash/wm/overview/window_selector.cc:410: observed_windows_.erase(window);
I could alternately move this line up before the early exit to guarantee that we
don't call RemoveObserver on any observed window which is destroyed, but it
should be in the windows_ list so this seems more like a workaround.

[James Cook, 2013-11-22 22:51:34]

LGTM. Adding CHECKs seems like the better path.

[commit-bot: I haz the power, 2013-11-22 23:28:02]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/flackr@chromium.org/83363004/1

[commit-bot: I haz the power, 2013-11-23 21:08:45]

Change committed as 236976