This happens to me when I tested a downloader extension from webstore, trying to download a file from the internetz.

It seems DownloadsInternalDeterminFilenameFunction will always fail a DCHECK because it never actually responds (from ExtensionFunction's pov, never calls SendResponse) before the function is destroyed:

[29818:29818:0223/154557.569987:FATAL:extension_function.cc(505)] Check failed: !browser_client || browser_client->IsShuttingDown() || did_respond() || ignore_all_did_respond_for_testing_do_not_use. downloadsInternal.determineFilename

bool DownloadsInternalDetermineFilenameFunction::RunAsync() {
  ...
  // This returns true/false.
  return ExtensionDownloadsEventRouter::DetermineFilename(
      GetProfile(),
      include_incognito(),
      extension()->id(),
      params->download_id,
      base::FilePath(filename),
      extensions::api::downloads::ParseFilenameConflictAction(
          params->conflict_action),
      &error_);
}

I think the function meant to call
SendResponse(ExtensionDownloadsEventRouter::DetermineFilename(...));
return true;

I have a quick fix for this.