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

mojo/public/cpp/bindings/string_traits.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_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();
+  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_;
 };
 
 // This must be specialized for any UserType to be serialized/deserialized as
 // a mojom string.
 //
 // TODO(yzshen): This is work in progress. Add better documentation once the
 // interface becomes more stable.
 template <typename UserType>
 struct StringTraits;
 
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_STRING_TRAITS_H_