Fix crashes in ui::Compositor on Mac.

Fix crashes in ui::Compositor on Mac.

Ensure that all ui::CompositorObserver instances are unhooked from
their ui::Compositor when the DelegatedFrameHost is destroyed.

This was not being done in instances where
RenderWidgetHostViewMac::Destroy was not being called before the
destructor RenderWidgetHostViewMac::~RenderWidgetHostViewMac.

BUG=401630
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=288895

[ccameron, 2014-08-11 23:21:44]

Simple fix, should be the last critical one for ui::Compositor on Mac!

[jbauman, 2014-08-11 23:23:02]

lgtm

[ccameron, 2014-08-11 23:28:16]

Thanks!

[ccameron, 2014-08-11 23:28:19]

The CQ bit was checked by ccameron@chromium.org

[commit-bot: I haz the power, 2014-08-11 23:32:03]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/ccameron@chromium.org/466483003/1

[commit-bot: I haz the power, 2014-08-12 00:23:41]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  mac_chromium_rel_swarming on tryserver.chromium.mac
(http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_rel_...)

[commit-bot: I haz the power, 2014-08-12 03:17:30]

Change committed as 288895