Remove DCHECK from xr_frame_provider

Replaces a flaky check in xr_frame_provider with an early return since
it's a valid state to be in, e.g. when exiting presentation.

Bug: 831243
Change-Id: Ie506095b5b5a1083ad669a000857a70b40f42a60
Reviewed-on: https://chromium-review.googlesource.com/1007622Reviewed-by: Klaus Weidner <klausw@chromium.org>
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550051}

third_party/blink/renderer/modules/xr/xr_frame_provider.cc

@@ -323,7 +323,8 @@ void XRFrameProvider::ProcessScheduledFrame(double timestamp) {
 void XRFrameProvider::SubmitWebGLLayer(XRWebGLLayer* layer, bool was_changed) {
   DCHECK(layer);
   DCHECK(layer->session() == exclusive_session_);
-  DCHECK(presentation_provider_);
+  if (!presentation_provider_.is_bound())
+    return;
 
   TRACE_EVENT1("gpu", "XRFrameProvider::SubmitWebGLLayer", "frame", frame_id_);