sync compositor: pass simple gfx types by const ref

See bug for reasoning

BUG=159273

Review URL: https://codereview.chromium.org/1417893006

Cr-Commit-Position: refs/heads/master@{#356653}
void AwContents::CancelMIDISysexPermissionRequests(const GURL& origin) {
  permission_request_handler_->CancelRequest(
      origin, AwPermissionRequest::AwPermissionRequest::MIDISysex);
}

void AwContents::CancelMIDISysexPermissionRequests(const GURL& origin) {
  permission_request_handler_->CancelRequest(
      origin, AwPermissionRequest::AwPermissionRequest::MIDISysex);
}
