Resubmit: Remove ExtensionService Garbage-Collecting methods.

Resubmit: Remove ExtensionService Garbage-Collecting methods.

Was: https://codereview.chromium.org/204983020/

This was reverted because of WeakPtr threading issues, which were sadly not
caught in testing. The issues have been resolved (we no longer use a weak ptr
on a separate thread), and the tests have been modified to use multiple threads
to more closely resemble reality.

BUG=351891
BUG=357753
TBR=rkc@chromium.org (for c/b/chromeos/kiosk_mode)

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=260998

[Devlin, 2014-03-28 22:42:43]

https://codereview.chromium.org/216553012/diff/1/chrome/browser/extensions/ex...
File chrome/browser/extensions/extension_garbage_collector.cc (right):

https://codereview.chromium.org/216553012/diff/1/chrome/browser/extensions/ex...
chrome/browser/extensions/extension_garbage_collector.cc:98: void
GarbageCollectExtensionsOnFileThread(
Difference 1: We now call this without a WeakPtr to ExtensionGarbageCollector,
and just pass in the install dir.

https://codereview.chromium.org/216553012/diff/1/chrome/browser/extensions/ex...
File chrome/browser/extensions/extension_garbage_collector_unittest.cc (right):

https://codereview.chromium.org/216553012/diff/1/chrome/browser/extensions/ex...
chrome/browser/extensions/extension_garbage_collector_unittest.cc:33: // TODO
(rdevlin.cronin): It's kind of hacky that we have to do these things
Difference 2: We now use multiple threads when we run this, to ensure we're
being safe.

[Yoyo Zhou, 2014-03-28 22:54:45]

LGTM

[Devlin, 2014-04-01 17:14:46]

The CQ bit was checked by rdevlin.cronin@chromium.org

[commit-bot: I haz the power, 2014-04-01 17:15:13]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/rdevlin.cronin@chromium.org/216553012/...

[commit-bot: I haz the power, 2014-04-01 17:36:11]

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

[commit-bot: I haz the power, 2014-04-01 17:36:12]

Try jobs failed on following builders:
  tryserver.chromium on linux_chromium_chromeos_rel

[Devlin, 2014-04-01 18:33:15]

The CQ bit was checked by rdevlin.cronin@chromium.org

[commit-bot: I haz the power, 2014-04-01 18:33:39]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/rdevlin.cronin@chromium.org/216553012/...

[commit-bot: I haz the power, 2014-04-01 22:37:43]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/rdevlin.cronin@chromium.org/216553012/...

[commit-bot: I haz the power, 2014-04-02 00:30:47]

Change committed as 260998