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

mojo/public/cpp/bindings/string_traits.h

Index: mojo/public/cpp/bindings/string_traits.h
diff --git a/mojo/public/cpp/bindings/string_traits.h b/mojo/public/cpp/bindings/string_traits.h
index c046854c2fb2b39acba1471ac2d91fd2f0d1060c..f0c4275c26497b3df8396efd866c05c8a4901078 100644
--- a/mojo/public/cpp/bindings/string_traits.h
+++ b/mojo/public/cpp/bindings/string_traits.h
@@ -5,8 +5,34 @@
 #ifndef MOJO_PUBLIC_CPP_BINDINGS_STRING_TRAITS_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_STRING_TRAITS_H_
 
+#include "base/logging.h"
+#include "mojo/public/cpp/bindings/lib/array_internal.h"
+
 namespace mojo {
 
+// Access to a string inside a mojo message.
+class StringDataView {
+ public:
+  explicit StringDataView(internal::String_Data* data) : data_(data) {}
+
+  // Whether the data represents a null string.
+  // Note: Must not access the following methods if this method returns true.
+  bool is_null() const { return !data_; }
+
+  const char* storage() const {
+    DCHECK(!is_null());
+    return data_->storage();
+  }
+
+  size_t size() const {
+    DCHECK(!is_null());
+    return data_->size();
+  }
+
+ private:
+  internal::String_Data* data_;
+};
+
 // This must be specialized for any UserType to be serialized/deserialized as
 // a mojom string.
 //