Wipe out QuotaThreadTask.

This is a one of a series of refactoring patches for QuotaManager.
http://codereview.chromium.org/10872054/
http://codereview.chromium.org/10917060/

BUG=139270

Review URL: https://chromiumcodereview.appspot.com/10919070

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@154987 0039d316-1c4b-4281-b951-d872f2087c98
  GlobalUsageCallback NewWaitableGlobalUsageCallback() {
    ++waiting_callbacks_;
    return base::Bind(&UsageAndQuotaDispatcherTask::DidGetGlobalUsage,
                      weak_factory_.GetWeakPtr());
  }

  GlobalUsageCallback NewWaitableGlobalUsageCallback() {
    ++waiting_callbacks_;
    return base::Bind(&UsageAndQuotaDispatcherTask::DidGetGlobalUsage,
                      weak_factory_.GetWeakPtr());
  }
