Fix shutdown assertions in WebViewImpl

Fix shutdown assertions in WebViewImpl

It's possible for some calls (e.g. WebViewImpl::layout) to try to
initialize compositing after shutdown.

This just moves an early out earlier so that it does not assert when
this happens.

This is a small band-aid fix to stop this assert from happening, rather
than the larger fix of trying to stop all the cases where this happen or
refactoring WebViewImpl to not have this awkwardness int he first place.

R=chrishtr@chromium.org
BUG=none

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=179860

[chrishtr, 2014-08-07 20:21:19]

lgtm

[commit-bot: I haz the power, 2014-08-07 20:22:44]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/enne@chromium.org/453523002/1

[commit-bot: I haz the power, 2014-08-07 22:04:16]

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

[enne (OOO), 2014-08-07 22:10:33]

+jamesr for Source/web OWNERZ

[commit-bot: I haz the power, 2014-08-07 22:33:24]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-08-07 22:33:45]

Try jobs failed on following builders:
  blink_presubmit on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/blink_presubmit/builds/1...)

[jamesr, 2014-08-07 22:43:10]

lgtm

[enne (OOO), 2014-08-08 19:19:23]

The CQ bit was checked by enne@chromium.org

[commit-bot: I haz the power, 2014-08-08 19:19:39]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/enne@chromium.org/453523002/1

[commit-bot: I haz the power, 2014-08-08 19:39:24]

Change committed as 179860