[NativeFS] Fix clusterfuzz crash in FileSystemChooser.

In content_shell --run-web-tests on linux there is no SelectFileDialog
implementation. Deal with this by aborting the dialog if we failed to
create one.

Bug: 1027908
Change-Id: Ibf4d3f1f91a719945140d88deb89dd3218efda36
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2314808
Commit-Queue: Victor Costan <pwnall@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791115}

content/browser/native_file_system/file_system_chooser.cc

@@ -147,6 +147,14 @@ void FileSystemChooser::CreateAndShow(
   listener->dialog_ = ui::SelectFileDialog::Create(
       listener,
       GetContentClient()->browser()->CreateSelectFilePolicy(web_contents));
+
+  // In content_shell --run-web-tests, there might be no dialog available. In
+  // that case just abort.
+  if (!listener->dialog_) {
+    listener->FileSelectionCanceled(nullptr);
+    return;
+  }
+
   // TODO(https://crbug.com/878581): Better/more specific options to pass to
   //     SelectFile.