Delete GenericChangeProcessor synchronously when possible.

Delete GenericChangeProcessor synchronously when possible.

ShareChangeProcessor is responsible for deleting GenericChangeProcessor.
Because ShareChangeProcessor's dtor may be invoked by either the UI
thread or the model type thread, it must take care to ensure
GenericChangeProcessor is destroyed on the model type thread.

For some model types, the model type thread *is* the UI thread.  In this
case it's preferable to destroy the GenericChangeProcessor directly in
ShareChangeProcessor's dtor.  Why does it matter?  It matters because we
want to ensure a model type's GenericChangeProcessor and its resources
have been destroyed before the model type completes its
KeyedService::Shutdown method.

BUG=425781
Committed: https://crrev.com/043302b06fcd0d0d18132db3b67df6105b54cf8e
Cr-Commit-Position: refs/heads/master@{#300720}

[maniscalco, 2014-10-22 16:20:22]

maniscalco@chromium.org changed reviewers:
+ zea@chromium.org

[maniscalco, 2014-10-22 16:20:23]

Nicolas, would you please review this change?

[Nicolas Zea, 2014-10-22 17:08:18]

lgtm

[maniscalco, 2014-10-22 17:08:52]

On 2014/10/22 17:08:18, Nicolas Zea wrote:
> lgtm

Thanks for the speedy review!

[maniscalco, 2014-10-22 17:08:55]

The CQ bit was checked by maniscalco@chromium.org

[commit-bot: I haz the power, 2014-10-22 17:11:07]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/671603005/1

[commit-bot: I haz the power, 2014-10-22 18:42:53]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-22 18:44:47]

Patchset 1 (id:??) landed as
https://crrev.com/043302b06fcd0d0d18132db3b67df6105b54cf8e
Cr-Commit-Position: refs/heads/master@{#300720}