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

ui/gfx/mojo/transform_struct_traits.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 UI_GFX_MOJO_TRANSFORM_STRUCT_TRAITS_H_
 #define UI_GFX_MOJO_TRANSFORM_STRUCT_TRAITS_H_
 
 #include "mojo/public/cpp/bindings/array_traits.h"
 #include "ui/gfx/mojo/transform.mojom.h"
 #include "ui/gfx/transform.h"
 
 namespace mojo {
 
 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) {
     return input.getFloat(static_cast<int>(index % 4),
                           static_cast<int>(index / 4));
   }
 };
 
 template <>
 struct StructTraits<gfx::mojom::Transform, gfx::Transform> {
   static const SkMatrix44& matrix(const gfx::Transform& transform) {
     return transform.matrix();
   }
 
   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;
   }
 };
 
 }  // namespace mojo
 
 #endif  // UI_GFX_MOJO_TRANSFORM_STRUCT_TRAITS_H_