Suffix VrShell calls with which thread they're expected to be run on.

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 edd2604b52fc32c11a258ed2732dcc4726a76a16..dd39ac5ed77e8843d1febf6f11b63e74e906d2c4 100644
--- a/chrome/browser/android/vr_shell/vr_web_contents_observer.cc
+++ b/chrome/browser/android/vr_shell/vr_web_contents_observer.cc
@@ -52,11 +52,11 @@ void VrWebContentsObserver::DidToggleFullscreenModeForTab(
 }
 
 void VrWebContentsObserver::WebContentsDestroyed() {
-  vr_shell_->ContentWebContentsDestroyed();
+  vr_shell_->ContentWebContentsDestroyedOnUI();
 }
 
 void VrWebContentsObserver::WasHidden() {
-  vr_shell_->ContentWasHidden();
+  vr_shell_->ContentWasHiddenOnUI();
 }
 
 }  // namespace vr_shell