ui/gfx mojo: Don't serialize the identity matrix

ui/gfx/mojo/transform_struct_traits.h

Index: ui/gfx/mojo/transform_struct_traits.h
diff --git a/ui/gfx/mojo/transform_struct_traits.h b/ui/gfx/mojo/transform_struct_traits.h
index a55122e40733067214ba5c6175ea5f06056df320..d3fb630480257574ee43f5bf3871cf9ddda2c334 100644
--- a/ui/gfx/mojo/transform_struct_traits.h
+++ b/ui/gfx/mojo/transform_struct_traits.h
@@ -15,6 +15,8 @@ template <>
 struct ArrayTraits<SkMatrix44> {
   using Element = float;
 
+  static bool IsNull(const SkMatrix44& input) { return input.isIdentity(); }

[Ian Vollick, 2016/08/10 15:02:34]

I'd prefer IsIdentity.

[Ian Vollick, 2016/08/10 15:13:57]

Fady explained to me offline about how IsNull interacts with the optional.
Disregard my comment. :)

[Fady Samuel, 2016/08/10 15:19:14]

Mojo calls IsNull to determine whether a given field should be serialized or
not.

+
   static size_t GetSize(const SkMatrix44& input) { return 16; }
 
   static float GetAt(const SkMatrix44& input, size_t index) {
@@ -32,6 +34,10 @@ struct StructTraits<gfx::mojom::Transform, gfx::Transform> {
   static bool Read(gfx::mojom::TransformDataView data, gfx::Transform* out) {
     ArrayDataView<float> matrix;
     data.GetMatrixDataView(&matrix);
+    if (matrix.is_null()) {
+      out->MakeIdentity();
+      return true;
+    }
     out->matrix().setColMajorf(matrix.data());
     return true;
   }