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

chrome/browser/android/vr_shell/vr_web_contents_observer.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/android/vr_shell/vr_web_contents_observer.h"
 
 #include "chrome/browser/android/vr_shell/ui_interface.h"
 #include "chrome/browser/android/vr_shell/vr_shell.h"
 #include "content/public/browser/navigation_handle.h"
 
@@ skipping 34 matching lines @@
     content::NavigationHandle* navigation_handle) {
   ui_interface_->SetURL(navigation_handle->GetURL());
 }
 
 void VrWebContentsObserver::DidToggleFullscreenModeForTab(
     bool entered_fullscreen, bool will_cause_resize) {
   ui_interface_->SetFullscreen(entered_fullscreen);
 }
 
 void VrWebContentsObserver::WebContentsDestroyed() {
-  vr_shell_->ContentWebContentsDestroyed();
+  vr_shell_->ContentWebContentsDestroyedOnUI();
 }
 
 void VrWebContentsObserver::WasHidden() {
-  vr_shell_->ContentWasHidden();
+  vr_shell_->ContentWasHiddenOnUI();
 }
 
 }  // namespace vr_shell