Clean up threading around VR Scene updates.

chrome/browser/android/vr_shell/vr_shell_gl.cc

Index: chrome/browser/android/vr_shell/vr_shell_gl.cc
diff --git a/chrome/browser/android/vr_shell/vr_shell_gl.cc b/chrome/browser/android/vr_shell/vr_shell_gl.cc
index 16c061416795471028b1da0a8793614b395b9137..9be9b9196ec936b3c76b70231f33f76339c6b242 100644
--- a/chrome/browser/android/vr_shell/vr_shell_gl.cc
+++ b/chrome/browser/android/vr_shell/vr_shell_gl.cc
@@ -169,6 +169,11 @@ int GetPixelEncodedPoseIndexByte() {
   return -1;
 }
 
+int64_t TimeInMicroseconds() {
+  return std::chrono::duration_cast<std::chrono::microseconds>(
+      std::chrono::steady_clock::now().time_since_epoch()).count();
+}
+
 }  // namespace
 
 VrShellGl::VrShellGl(
@@ -630,7 +635,7 @@ void VrShellGl::DrawFrame() {
 
   // Update the render position of all UI elements (including desktop).
   const float screen_tilt = kDesktopScreenTiltDefault * M_PI / 180.0f;
-  scene_->UpdateTransforms(screen_tilt, UiScene::TimeInMicroseconds());
+  scene_->UpdateTransforms(screen_tilt, TimeInMicroseconds());
 
   UpdateController(GetForwardVector(head_pose));
 
@@ -983,4 +988,8 @@ void VrShellGl::ForceExitVr() {
       FROM_HERE, base::Bind(&VrShell::ForceExitVr, weak_vr_shell_));
 }
 
+void VrShellGl::UpdateScene(std::unique_ptr<base::ListValue> commands) {
+  scene_->HandleCommands(std::move(commands), TimeInMicroseconds());
+}
+
 }  // namespace vr_shell