[VR Shell] Fix hidden elements being hit-testable.

chrome/browser/android/vr_shell/ui_elements.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/ui_elements.h"
 
 #include <limits>
 
 #include "base/logging.h"
 #include "chrome/browser/android/vr_shell/animation.h"
@@ skipping 178 matching lines @@
       ++it;
     }
   }
 }
 
 bool ContentRectangle::IsVisible() const {
   return visible && computed_opacity > 0.0f;
 }
 
 bool ContentRectangle::IsHitTestable() const {
-  return hit_testable;
+  return IsVisible() && hit_testable;
 }
 
 }  // namespace vr_shell