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

services/ui/public/interfaces/animations.mojom

Index: services/ui/public/interfaces/animations.mojom
diff --git a/services/ui/public/interfaces/animations.mojom b/services/ui/public/interfaces/animations.mojom
index eae16ea32d5e0088c9fc32e0b6ef4f8fecd01899..983e417f463d4178d4bc545a1d50afcbd33802c0 100644
--- a/services/ui/public/interfaces/animations.mojom
+++ b/services/ui/public/interfaces/animations.mojom
@@ -22,7 +22,7 @@ enum AnimationProperty {
 
 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