Control the VrShell content quad via the HTML UI.

chrome/browser/android/vr_shell/ui_elements.h

Index: chrome/browser/android/vr_shell/ui_elements.h
diff --git a/chrome/browser/android/vr_shell/ui_elements.h b/chrome/browser/android/vr_shell/ui_elements.h
index fda2a2fa2b819d6ca2f106165eb13e621bfb77fe..056896dcaf8ffe5c792d779ae647cc0cdc161c58 100644
--- a/chrome/browser/android/vr_shell/ui_elements.h
+++ b/chrome/browser/android/vr_shell/ui_elements.h
@@ -80,6 +80,10 @@ struct ContentRectangle : public WorldRectangle {
   // rather than the world.
   bool lock_to_fov = false;
 
+  // If true, this element is the content quad. Only one content quad may be
+  // added to the scene.
+  bool content_quad = false;
+
   // Specifies the region (in pixels) of a texture to render.
   Recti copy_rect = {0, 0, 0, 0};