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

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 f0c4275c26497b3df8396efd866c05c8a4901078..cd3a34f1d408fd86285089a59d63daa50a0fe2cd 100644
--- a/mojo/public/cpp/bindings/string_traits.h
+++ b/mojo/public/cpp/bindings/string_traits.h
@@ -13,21 +13,13 @@ 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();
+  explicit StringDataView(internal::String_Data* data) : data_(data) {
+    DCHECK(data_);
   }
 
-  size_t size() const {
-    DCHECK(!is_null());
-    return data_->size();
-  }
+  const char* storage() const { return data_->storage(); }
+
+  size_t size() const { return data_->size(); }
 
  private:
   internal::String_Data* data_;