Revert of Check to ensure PowerObservers are added and removed on the same thread

Revert of Check to ensure PowerObservers are added and removed on the same thread (patchset #7 id:120001 of https://codereview.chromium.org/502003003/)

Reason for revert:
Causing failures on Mac try jobs:
ExtensionServiceTest.ExternalExtensionAutoAcknowledgement (run #1):
[ RUN      ] ExtensionServiceTest.ExternalExtensionAutoAcknowledgement
[8427:1799:0904/211500:4724222818698:FATAL:power_monitor.cc(38)] Check failed: obs->power_monitor_thread_checker_.

Original issue's description:
> Check to ensure PowerObservers are added and removed on the same thread
> 
> This behavior is required by ObserverListThreadSafe and may leads to crashes if not followed
> 
> BUG=404767
> 
> Committed: https://chromium.googlesource.com/chromium/src/+/518c46fbba70c91169bfc0c64b542f06e56b3915

TBR=thakis@chromium.org,brettw@chromium.org,scottmg@chromium.org,bajones@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=404767
Committed: https://crrev.com/5b9e04fc7c694535d2cc21710d7f7c688700648f
Cr-Commit-Position: refs/heads/master@{#293448}

[dcheng, 2014-09-05 05:04:48]

Created Revert of Check to ensure PowerObservers are added and removed on the
same thread

[commit-bot: I haz the power, 2014-09-05 05:05:55]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/dcheng@chromium.org/544913003/1

[commit-bot: I haz the power, 2014-09-05 05:07:36]

Committed patchset #1 (id:1) as 4f4f127b8469a50847c5cb921a8846c15e58b6b6

[bajones, 2014-09-05 13:59:16]

On 2014/09/05 05:07:36, I haz the power (commit-bot) wrote:
> Committed patchset #1 (id:1) as 4f4f127b8469a50847c5cb921a8846c15e58b6b6

If this check was failing it means that the PowerObserver subclass in question
is doing something bad and may potentially cause flaky crashes. I'm OOO till
Tues, I'll try to re-land this then, but I would highly recommend that in the
meantime we find the right owner for the ExtensionService that was triggering
the check and have them ensure that the PowerObserver add/remove are happening
on the same thread.

[commit-bot: I haz the power, 2014-09-10 03:37:07]

Patchset 1 (id:??) landed as
https://crrev.com/5b9e04fc7c694535d2cc21710d7f7c688700648f
Cr-Commit-Position: refs/heads/master@{#293448}