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

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

Index: mojo/public/cpp/bindings/tests/rect_blink_traits.h
diff --git a/mojo/public/cpp/bindings/tests/rect_blink_traits.h b/mojo/public/cpp/bindings/tests/rect_blink_traits.h
index c4e4014fc64191286579263447242afe44bcacc7..30c93e136f015f8735f8197f5b9e38830509b1b0 100644
--- a/mojo/public/cpp/bindings/tests/rect_blink_traits.h
+++ b/mojo/public/cpp/bindings/tests/rect_blink_traits.h
@@ -18,8 +18,13 @@ struct StructTraits<test::blink::TypemappedRect, test::RectBlink> {
   static int width(const test::RectBlink& r) { return r.width(); }
   static int height(const test::RectBlink& r) { return r.height(); }
 
-  static bool Read(test::blink::TypemappedRect::Reader r,
+  static bool Read(test::blink::TypemappedRectDataView r,
                    test::RectBlink* out) {
+    if (r.is_null()) {
+      *out = test::RectBlink();
+      return true;
+    }
+
     if (r.x() < 0 || r.y() < 0 || r.width() < 0 || r.height() < 0) {
       return false;
     }