VrShell: Allow native control of UI element opacity.

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 a8a2527b004b06ed91238b6f34594d4773da4c60..a034e3db07440c3c5fbb0ca906e17fff0916d038 100644
--- a/chrome/browser/android/vr_shell/ui_elements.h
+++ b/chrome/browser/android/vr_shell/ui_elements.h
@@ -100,6 +100,12 @@ struct ContentRectangle : public WorldRectangle {
   // after rotation and scaling.
   gvr::Vec3f translation = {0.0f, 0.0f, 0.0f};
 
+  // The opacity of the object (between 0.0 and 1.0).
+  float opacity = 1.0f;
+
+  // The computed opacity, incorporating opacity of parent objects.
+  float computed_opacity;
+
   // If anchoring is specified, the translation will be relative to the
   // specified edge(s) of the parent, rather than the center.  A parent object
   // must be specified when using anchoring.