Clean up VR Shell mode transitions (and fix potential webvr startup race).

chrome/browser/android/vr_shell/vr_web_contents_observer.cc

Index: chrome/browser/android/vr_shell/vr_web_contents_observer.cc
diff --git a/chrome/browser/android/vr_shell/vr_web_contents_observer.cc b/chrome/browser/android/vr_shell/vr_web_contents_observer.cc
index 4b337eb261a914e200a973509edb330a7095f619..84165bd3865c8728f691ba705cce8e582f7dc693 100644
--- a/chrome/browser/android/vr_shell/vr_web_contents_observer.cc
+++ b/chrome/browser/android/vr_shell/vr_web_contents_observer.cc
@@ -46,13 +46,7 @@ void VrWebContentsObserver::DidFinishNavigation(
 void VrWebContentsObserver::DidToggleFullscreenModeForTab(
     bool entered_fullscreen, bool will_cause_resize) {
   // TODO(amp): Use will_cause_resize to signal ui of pending size changes.
-  int mode = ui_interface_->GetMode();
-  if (entered_fullscreen && mode == UiInterface::Mode::STANDARD) {
-    ui_interface_->SetMode(UiInterface::Mode::CINEMA);
-  }
-  if (!entered_fullscreen && mode == UiInterface::Mode::CINEMA) {
-    ui_interface_->SetMode(UiInterface::Mode::STANDARD);
-  }
+  ui_interface_->SetCinemaMode(entered_fullscreen);
 }
 
 }  // namespace vr_shell