Revert 233815 "Quota: Implement QuotaBackendImpl"

This breaks content_unittests on some bots.

http://build.chromium.org/p/chromium.linux/builders/Linux Aura/builds/11164

> Quota: Implement QuotaBackendImpl
> 
> This introduces QuotaBackendImpl working as bridge among FileSystemUsageCache,
> QuotaManager and QuotaReservationManager. The backend will be created by
> FileSystemContext and retained by QuotaReservationManager.
> 
> 
> BUG=303443
> TEST=content_unittests --gtest_filter=QuotaBackendImplTest.*
> TEST=content_unittests --gtest_filter=QuotaReservationManagerTest.*
> TBR=jochen@chromium.org
> 
> Review URL: https://codereview.chromium.org/61593002

TBR=nhiroki@chromium.org

Review URL: https://codereview.chromium.org/64883003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233828 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233828