Mojo C++ bindings: switch the existing usage of StructTraits to use the new data view interface.

mojo/public/cpp/bindings/tests/rect_chromium_traits.h

 // Copyright 2015 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 MOJO_PUBLIC_CPP_BINDINGS_TESTS_RECT_CHROMIUM_TRAITS_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_TESTS_RECT_CHROMIUM_TRAITS_H_
 
 #include "mojo/public/cpp/bindings/struct_traits.h"
 #include "mojo/public/cpp/bindings/tests/rect_chromium.h"
 #include "mojo/public/interfaces/bindings/tests/rect.mojom.h"
 
 namespace mojo {
 
 template <>
 struct StructTraits<test::TypemappedRect, test::RectChromium> {
   static int x(const test::RectChromium& r) { return r.x(); }
   static int y(const test::RectChromium& r) { return r.y(); }
   static int width(const test::RectChromium& r) { return r.width(); }
   static int height(const test::RectChromium& r) { return r.height(); }
 
-  static bool Read(test::TypemappedRect::Reader r, test::RectChromium* out) {
+  static bool Read(test::TypemappedRectDataView r, test::RectChromium* out) {
+    if (r.is_null()) {
+      *out = test::RectChromium();
+      return true;
+    }
+
     if (r.width() < 0 || r.height() < 0)
       return false;
     out->set_x(r.x());
     out->set_y(r.y());
     out->set_width(r.width());
     out->set_height(r.height());
     return true;
   }
 };
 
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_TESTS_RECT_CHROMIUM_TRAITS_H_