VrShell: Allow native control of UI element opacity.

chrome/browser/android/vr_shell/ui_elements.cc

Index: chrome/browser/android/vr_shell/ui_elements.cc
diff --git a/chrome/browser/android/vr_shell/ui_elements.cc b/chrome/browser/android/vr_shell/ui_elements.cc
index e0285885896ec56991e27e350ac56c73c8993a17..298bc7007f87a5878f11f274db80a764963b777e 100644
--- a/chrome/browser/android/vr_shell/ui_elements.cc
+++ b/chrome/browser/android/vr_shell/ui_elements.cc
@@ -122,6 +122,9 @@ void ContentRectangle::Animate(int64_t time) {
           animation.from.push_back(translation.y);
           animation.from.push_back(translation.z);
           break;
+        case Animation::OPACITY:
+          animation.from.push_back(opacity);
+          break;
       }
     }
     CHECK_EQ(animation.from.size(), animation.to.size());
@@ -170,6 +173,10 @@ void ContentRectangle::Animate(int64_t time) {
         translation.y = values[1];
         translation.z = values[2];
         break;
+      case Animation::OPACITY:
+        CHECK_EQ(animation.from.size(), 1u);
+        opacity = values[0];
+        break;
     }
   }
   for (auto it = animations.begin(); it != animations.end();) {