Don't shut down OpenVR render loop on connection errors

A normal immersive session shutdown triggers this error handler,
and stopping the render loop lost a pending gamepad_callback_,
so future immersive sessions weren't seeing the controllers
anymore. Instead, keep the render loop running, it'll be cleaned
up when exiting the page entirely.

BUG=875187

Cq-Include-Trybots: luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I89a333a9cd104069d6e228248608957bec665a91
Reviewed-on: https://chromium-review.googlesource.com/1194252Reviewed-by: Bill Orr <billorr@chromium.org>
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587130}

device/vr/openvr/openvr_device.cc

@@ -264,7 +264,10 @@ void OpenVRDevice::OnPresentingControllerMojoConnectionError() {
   render_loop_->task_runner()->PostTask(
       FROM_HERE,
       base::Bind(&OpenVRRenderLoop::ExitPresent, render_loop_->GetWeakPtr()));
-  render_loop_->Stop();
+  // Don't stop the render loop here. We need to keep the gamepad provider alive
+  // so that we don't lose a pending mojo gamepad_callback_.
+  // TODO(https://crbug.com/875187): Alternatively, we could recreate the
+  // provider on the next session, or look into why the callback gets lost.
   OnExitPresent();
   exclusive_controller_binding_.Close();
 }