Add null check when SandboxFSWriter write fails

On write failure, the QuotaManagerProxy calls NotifyWriteFailed to
surface this failure to quota.

On filesystems which don't support quota, no check is currently
made that the manager exists before NotifyWriteFailed is called.

Bug: N/A
Change-Id: Iae14483fb9c395576102c82c3a5fa60fac9f1343
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2593745
Auto-Submit: Austin Sullivan <asully@chromium.org>
Commit-Queue: Austin Sullivan <asully@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837642}

storage/browser/file_system/sandbox_file_stream_writer.cc

@@ -226,8 +226,11 @@ void SandboxFileStreamWriter::DidWrite(int write_response) {
   if (write_response <= 0) {
     // TODO(crbug.com/1091792): Consider listening explicitly for out
     // of space errors instead of surfacing all write errors to quota.
-    file_system_context_->quota_manager_proxy()->NotifyWriteFailed(
-        url_.origin());
+    QuotaManagerProxy* quota_manager_proxy =
+        file_system_context_->quota_manager_proxy();
+    if (quota_manager_proxy) {
+      quota_manager_proxy->NotifyWriteFailed(url_.origin());
+    }
     if (CancelIfRequested())
       return;
     std::move(write_callback_).Run(write_response);