Mojo C++ bindings: support mapping T and T? differently.

services/ui/public/interfaces/animations.mojom

 // 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.
 
 module ui.mojom;
 
 import "ui/gfx/mojo/transform.mojom";
 
 enum AnimationTweenType {
   LINEAR,
   EASE_IN,
   EASE_OUT,
   EASE_IN_OUT,
 };
 
 enum AnimationProperty {
   // Used for pausing.
   NONE,
   OPACITY,
   TRANSFORM,
 };
 
 struct AnimationValue {
   float float_value;
-  gfx.mojom.Transform? transform;
+  gfx.mojom.Transform transform;

[sky, 2016/07/26 21:26:16]

This structure is like a union, so we want to support '?'. Is there a reason we
can't allow it?

[yzshen1, 2016/07/26 21:32:36]

Sure we can do that.
I made this change because the code never took advantage of nullable:  prior to
this CL the field was mapped to gfx::Transform and according to how the
StructTraits is written, it's impossible to encode gfx::Transform as null.

With this change, we will map gfx.mojom.Transform? to
base::Optional<gfx::Transform>. If you think that is useful, I can keep it as
nullable and modify the code accordingly.

WDYT?

 };
 
 // Identifies how a particular property should be animated between a start and
 // target value.
 struct AnimationElement {
   AnimationProperty property;
 
   // Duration is in microseconds.
   int64 duration;
 
@@ skipping 16 matching lines @@
 
   array<AnimationElement> elements;
 };
 
 // AnimationGroup identifies a window and a set of AnimationSequences to apply
 // to the window. Each sequence is run in parallel.
 struct AnimationGroup {
   uint32 window_id;
   array<AnimationSequence> sequences;
 };