Register mixed_reality_renderloop as a UI Thread

On windows release 1809 (and presumably up), input was not being routed
to the chromium webxr process/threads.  This resulted in polling tracked
controllers and subscriptions to input events never firing or returning
any controllers.  This change has no effect on 1803.

Bug: 953129
Change-Id: Ifdfbd47906b1d5817e791449ac94235e346ea0e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1577485
Commit-Queue: Alexander Cooper <alcooper@chromium.org>
Reviewed-by: Bill Orr <billorr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652913}

device/vr/windows_mixed_reality/mixed_reality_device.cc

@@ -9,6 +9,7 @@
 
 #include "base/macros.h"
 #include "base/memory/ptr_util.h"
+#include "base/message_loop/message_loop.h"
 #include "base/numerics/math_constants.h"
 #include "base/threading/thread.h"
 #include "build/build_config.h"
@@ -94,7 +95,11 @@ void MixedRealityDevice::RequestSession(
     CreateRenderLoop();
 
   if (!render_loop_->IsRunning()) {
-    render_loop_->Start();
+    // We need to start a UI message loop or we will not receive input events
+    // on 1809 or newer.
+    base::Thread::Options options;
+    options.message_loop_type = base::MessageLoop::TYPE_UI;
+    render_loop_->StartWithOptions(options);
 
     // IsRunning() should be true here unless the thread failed to start (likely
     // memory exhaustion). If the thread fails to start, then we fail to create