Mac: Fix crash due to occluding view during tear-down

Mac: Fix crash due to occluding view during tear-down

WebContentsViewMac will get a call during WebContentsImpl's destructor,
indicating that the contents are being occluded. It is not very helpful
to forward this to the WebContentsImpl (because it's being destroyed
anyway), and is dangerous in that the WebContentsImpl or its
sub-structures may be in an inconsistent state and not expecting to be
called in to.

In particular, in the bug, we were destroying the FrameTree high in
the stack, and then trying to iterate over it lower in the stack.

BUG=429993
Committed: https://crrev.com/f128f129f90a6baf20e2dd30e3ce55f30d3d8a0d
Cr-Commit-Position: refs/heads/master@{#315430}

[ccameron, 2015-02-09 21:44:51]

ccameron@chromium.org changed reviewers:
+ avi@chromium.org

[ccameron, 2015-02-09 21:44:51]

Small fix for browser crash issue.

[Avi (use Gerrit), 2015-02-09 21:57:43]

lgtm

[ccameron, 2015-02-09 22:23:46]

Thanks!

[ccameron, 2015-02-09 22:23:49]

The CQ bit was checked by ccameron@chromium.org

[commit-bot: I haz the power, 2015-02-09 22:25:06]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/910013003/1

[commit-bot: I haz the power, 2015-02-09 23:28:57]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-09 23:29:28]

Patchset 1 (id:??) landed as
https://crrev.com/f128f129f90a6baf20e2dd30e3ce55f30d3d8a0d
Cr-Commit-Position: refs/heads/master@{#315430}