Unset CompositorWorkerGlobalScope during WorkerGlobalScope::dipose.

Unset CompositorWorkerGlobalScope during WorkerGlobalScope::dipose.

We will still generate compositor frames while shutting down the compositor
worker global scope, so we need to ensure we don't try to run our animation
callbacks on the now disposed worker thread.

BUG=645991
TEST=CompositorWorkerThreadTest.Shutdown

[flackr, 2016-09-12 22:35:21]

flackr@chromium.org changed reviewers:
+ vollick@chromium.org

[flackr, 2016-09-12 22:35:22]

Hey Ian, can you take a look at this simple patch. This ensures that after we
dispose of the global scope we no longer try to execute animation frame
callbacks even if they had been registered.

[Ian Vollick, 2016-09-12 23:41:02]

On 2016/09/12 22:35:22, flackr wrote:
> Hey Ian, can you take a look at this simple patch. This ensures that after we
> dispose of the global scope we no longer try to execute animation frame
> callbacks even if they had been registered.

I like it. lgtm.

[flackr, 2016-09-13 14:54:58]

flackr@chromium.org changed reviewers:
+ bokan@chromium.org

[flackr, 2016-09-13 14:54:58]

+bokan, can you give the OWNERS stamp for Source/web?

[bokan, 2016-09-13 15:00:54]

lgtm

[flackr, 2016-09-13 15:06:14]

The CQ bit was checked by flackr@chromium.org

[commit-bot: I haz the power, 2016-09-13 15:06:35]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-09-13 15:09:16]

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

[commit-bot: I haz the power, 2016-09-13 15:09:17]

Try jobs failed on following builders:
  mac_chromium_compile_dbg_ng on master.tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_comp...)