Mojo C++ bindings: expose public <struct>DataView and StringDataView.

mojo/public/cpp/bindings/string_traits_string_piece.h

Index: mojo/public/cpp/bindings/string_traits_string_piece.h
diff --git a/mojo/public/cpp/bindings/string_traits_string_piece.h b/mojo/public/cpp/bindings/string_traits_string_piece.h
index cc126edbc1f07d26fb5a1850a322c9187248888c..b6018f22fbe8a99d8da775620025aebf84bfb347 100644
--- a/mojo/public/cpp/bindings/string_traits_string_piece.h
+++ b/mojo/public/cpp/bindings/string_traits_string_piece.h
@@ -25,11 +25,9 @@ struct StringTraits<base::StringPiece> {
     return input.data();
   }
 
-  // TODO(yzshen): Use a public type, such as mojo::String::DataView, for
-  // |input|.
-  static bool Read(internal::String_Data* input, base::StringPiece* output) {
-    if (input)
-      output->set(input->storage(), input->size());
+  static bool Read(StringDataView input, base::StringPiece* output) {
+    if (!input.is_null())
+      output->set(input.storage(), input.size());
     else
       output->set(nullptr, 0);
     return true;