Control the VrShell content quad via the HTML UI.

chrome/browser/resources/vr_shell/vr_shell_ui_api.js

Index: chrome/browser/resources/vr_shell/vr_shell_ui_api.js
diff --git a/chrome/browser/resources/vr_shell/vr_shell_ui_api.js b/chrome/browser/resources/vr_shell/vr_shell_ui_api.js
index e90757e343bf59fd9639aa12ffd0d8d8d2251774..a62e5cfbd4ddb2b8d722e44655a1d0fcdfa855aa 100644
--- a/chrome/browser/resources/vr_shell/vr_shell_ui_api.js
+++ b/chrome/browser/resources/vr_shell/vr_shell_ui_api.js
@@ -106,6 +106,10 @@ var api = (function() {
    */
   class UiElementUpdate {
 
+    setIsContentQuad() {
+      this.contentQuad = true;
+    }
+
     /**
      * Specify a parent for this element. If set, this element is positioned
      * relative to its parent element, rather than absolutely. This allows