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

mojo/public/cpp/bindings/string_traits_string_piece.h

 // Copyright 2016 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_STRING_TRAITS_STRING_PIECE_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_STRING_TRAITS_STRING_PIECE_H_
 
 #include "base/strings/string_piece.h"
 #include "mojo/public/cpp/bindings/string_traits.h"
 
 namespace mojo {
 
 template <>
 struct StringTraits<base::StringPiece> {
   // No IsNull() function, which means that base::StringPiece is always
   // considered as non-null mojom string. We could have let StringPiece
   // containing a null data pointer map to null mojom string, but
   // StringPiece::empty() returns true in this case. It seems confusing to mix
   // the concept of empty and null strings, especially because they mean
   // different things in mojom.
 
   static size_t GetSize(const base::StringPiece& input) { return input.size(); }
 
   static const char* GetData(const base::StringPiece& input) {
     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;
   }
 };
 
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_STRING_TRAITS_STRING_PIECE_H_