Quota double-delete fix

BUG=142310


Review URL: https://chromiumcodereview.appspot.com/10832407

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@152532 0039d316-1c4b-4281-b951-d872f2087c98
 void QuotaTask::CallCompleted() {
  DCHECK(original_task_runner_->BelongsToCurrentThread());
  if (observer_) {
    observer_->UnregisterTask(this);
    Completed();
  }
}

 void QuotaTask::CallCompleted() {
  DCHECK(original_task_runner_->BelongsToCurrentThread());
  if (observer_) {
    observer_->UnregisterTask(this);
    Completed();
  }
}
