VrShell: Allow native control of UI element opacity.

chrome/browser/android/vr_shell/animation.h

 // 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.
 
 #ifndef CHROME_BROWSER_ANDROID_VR_SHELL_ANIMATION_H_
 #define CHROME_BROWSER_ANDROID_VR_SHELL_ANIMATION_H_
 
 #include <memory>
 #include <vector>
 
 #include "base/macros.h"
 
 namespace vr_shell {
 
 namespace easing {
 class Easing;
 }
 
 // Describes the characteristics of a transition from an initial set of values
 // to a final set, with timing and interpolation information.  Other classes use
 // this information to animate UI element location, size, and other properties.
 class Animation {
  public:
   enum Property {
     COPYRECT = 0,
     SIZE,
     TRANSLATION,
     SCALE,
     ROTATION,
+    OPACITY,
   };
 
   Animation(int id,
             Property property,
             std::unique_ptr<easing::Easing> easing,
             std::vector<float> from,
             std::vector<float> to,
             int64_t start,
             int64_t duration);
   ~Animation();
 
   int id;
   Property property;
   std::unique_ptr<easing::Easing> easing;
   std::vector<float> from;
   std::vector<float> to;
   int64_t start;
   int64_t duration;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(Animation);
 };
 
 }  // namespace vr_shell
 
 #endif  // CHROME_BROWSER_ANDROID_VR_SHELL_ANIMATION_H_