Disallow task observers on some BrowserThreads.

Disallow task observers on some BrowserThreads.

This CL disallows task observers on the DB, FILE, FILE_USER_BLOCKING,
PROCESS_LAUNCHER and CACHE threads. Calling Add/RemoveTaskObserver
on these threads will result in a crash.

This is important because these threads will be migrated to
TaskScheduler which doesn't support task observers.

To our knowledge, no code uses task observers on these threads. This
CL will help us confirm that in the wild.

BUG=653916
Committed: https://crrev.com/dbed8d7df17c48f391fde23becf4cb926079610a
Cr-Commit-Position: refs/heads/master@{#425418}

[fdoray, 2016-10-13 15:51:46]

The CQ bit was checked by fdoray@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-10-13 15:52:12]

Dry run: 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, 2016-10-13 17:35:12]

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

[commit-bot: I haz the power, 2016-10-13 17:35:13]

Dry run: This issue passed the CQ dry run.

[fdoray, 2016-10-13 17:35:45]

fdoray@chromium.org changed reviewers:
+ jam@chromium.org

[fdoray, 2016-10-13 17:35:46]

PTAL

[jam, 2016-10-14 17:31:08]

lgtm

[fdoray, 2016-10-14 17:39:28]

The CQ bit was checked by fdoray@chromium.org

[commit-bot: I haz the power, 2016-10-14 17:39:54]

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, 2016-10-14 19:13:00]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2016-10-14 19:16:49]

Description was changed from

==========
Disallow task observers on some BrowserThreads.

This CL disallows task observers on the DB, FILE, FILE_USER_BLOCKING,
PROCESS_LAUNCHER and CACHE threads. Calling Add/RemoveTaskObserver
on these threads will result in a crash.

This is important because these threads will be migrated to
TaskScheduler which doesn't support task observers.

To our knowledge, no code uses task observers on these threads. This
CL will help us confirm that in the wild.

BUG=653916
==========

to

==========
Disallow task observers on some BrowserThreads.

This CL disallows task observers on the DB, FILE, FILE_USER_BLOCKING,
PROCESS_LAUNCHER and CACHE threads. Calling Add/RemoveTaskObserver
on these threads will result in a crash.

This is important because these threads will be migrated to
TaskScheduler which doesn't support task observers.

To our knowledge, no code uses task observers on these threads. This
CL will help us confirm that in the wild.

BUG=653916

Committed: https://crrev.com/dbed8d7df17c48f391fde23becf4cb926079610a
Cr-Commit-Position: refs/heads/master@{#425418}
==========

[commit-bot: I haz the power, 2016-10-14 19:16:50]

Patchset 1 (id:??) landed as
https://crrev.com/dbed8d7df17c48f391fde23becf4cb926079610a
Cr-Commit-Position: refs/heads/master@{#425418}