Mojo C++ bindings: provide data view for all object types.

mojo/public/cpp/bindings/struct_traits.h

Index: mojo/public/cpp/bindings/struct_traits.h
diff --git a/mojo/public/cpp/bindings/struct_traits.h b/mojo/public/cpp/bindings/struct_traits.h
index 0f0bea73dd7999b1569acdc989def10841b13ff8..b2a60c7887b9280d2772266bca0c3c25822e4497 100644
--- a/mojo/public/cpp/bindings/struct_traits.h
+++ b/mojo/public/cpp/bindings/struct_traits.h
@@ -55,7 +55,8 @@ namespace mojo {
 //        static bool Read(|MojomType|DataView data, T* output);
 //
 //      The generated |MojomType|DataView type provides a convenient,
-//      inexpensive view of a serialized struct's field data.
+//      inexpensive view of a serialized struct's field data. The caller
+//      guarantees that |!data.is_null()|.
 //
 //      Returning false indicates invalid incoming data and causes the message
 //      pipe receiving it to be disconnected. Therefore, you can do custom