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

Index: mojo/public/cpp/bindings/tests/rect_chromium_traits.h
diff --git a/mojo/public/cpp/bindings/tests/rect_chromium_traits.h b/mojo/public/cpp/bindings/tests/rect_chromium_traits.h
index 252de72d5172e77dc4afe03f2c2072f18eef591e..2c6671d103661b8c546672ff58d3dffe861c87c1 100644
--- a/mojo/public/cpp/bindings/tests/rect_chromium_traits.h
+++ b/mojo/public/cpp/bindings/tests/rect_chromium_traits.h
@@ -18,7 +18,12 @@ struct StructTraits<test::TypemappedRect, test::RectChromium> {
   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());