Disable FileSystemManager::CreateWriter if WritableFiles isn't enabled.

Bug: 922677
Change-Id: Ib16137cbabb2ec07f1ffc0484722f1d9cc533404
Reviewed-on: https://chromium-review.googlesource.com/c/1416570
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623552}
void FileSystemManagerImpl::DidFinish(
    base::OnceCallback<void(base::File::Error)> callback,
    base::File::Error error_code) {
  DCHECK_CURRENTLY_ON(BrowserThread::IO);
  std::move(callback).Run(error_code);
}

void FileSystemManagerImpl::DidFinish(
    base::OnceCallback<void(base::File::Error)> callback,
    base::File::Error error_code) {
  DCHECK_CURRENTLY_ON(BrowserThread::IO);
  std::move(callback).Run(error_code);
}
