Revert of Remove ExtensionService Garbage-Collecting methods.

Revert of Remove ExtensionService Garbage-Collecting methods. (https://codereview.chromium.org/204983020/)

Reason for revert:
Launch the browser and just wait a bit, eventually it crashes with:

[6912:6955:0328/131903:FATAL:weak_ptr.cc(26)] Check failed:
sequence_checker_.CalledOnValidSequencedThread(). WeakPtrs must be checked on
the same sequenced thread.

Commenting out that check then gets you to:

[12009:12077:0328/133048:FATAL:extension_garbage_collector.cc(232)] Check
failed: content::BrowserThread::CurrentlyOn(content::BrowserThread::FILE). 

Reverting the patch fixes the issue.

Original issue's description:
> Remove ExtensionService Garbage-Collecting methods.
> 
> Make a new class, ExtensionGarbageCollector, which performs:
> - ExtensionService::GarbageCollectExtensions(),
> - extension_file_util::GarbageCollectExtensions() (the file-thread impl of
>   ExtensionService::GarbageCollectExtensions()),
> - ExtensionService::GarbageCollectIsolatedStorage()
> 
> BUG=351891
> TBR=rkc@chromium.org (for c/b/chromeos/kiosk_mode - no functional changes, but the TODO has moved from ExtensionService to ExtensionGarbageCollector).
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=260207

TBR=yoz@chromium.org,jyasskin@chromium.org,rkc@chromium.org,rdevlin.cronin@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=351891
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=260265

[DaleCurtis, 2014-03-28 20:40:21]

Created Revert of Remove ExtensionService Garbage-Collecting methods.

[commit-bot: I haz the power, 2014-03-28 20:41:13]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/dalecurtis@chromium.org/217233004/1

[commit-bot: I haz the power, 2014-03-28 20:43:42]

Change committed as 260265

[DaleCurtis, 2014-03-28 20:46:09]

If waiting doesn't trigger the crash, try loading this video:

http://commondatastorage.googleapis.com/dalecurtis-shared/buck2.mp4