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

mojo/public/cpp/bindings/lib/string_traits_wtf.cc

 // 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.
 
 #include "mojo/public/cpp/bindings/string_traits_wtf.h"
 
 #include <string.h>
 
 #include "base/logging.h"
 #include "mojo/public/cpp/bindings/lib/array_internal.h"
@@ skipping 25 matching lines @@
 #if DCHECK_IS_ON()
   DCHECK_EQ(adaptor->original_size_in_bytes,
             static_cast<size_t>(input.sizeInBytes()));
 #endif
   return adaptor;
 }
 
 }  // namespace
 
 // static
+void StringTraits<WTF::String>::SetToNull(WTF::String* output) {
+  if (output->isNull())
+    return;
+
+  WTF::String result;
+  output->swap(result);
+}
+
+// static
 void* StringTraits<WTF::String>::SetUpContext(const WTF::String& input) {
   return new UTF8AdaptorInfo(input);
 }
 
 // static
 void StringTraits<WTF::String>::TearDownContext(const WTF::String& input,
                                                 void* context) {
   delete ToAdaptor(input, context);
 }
 
 // static
 size_t StringTraits<WTF::String>::GetSize(const WTF::String& input,
                                           void* context) {
   return ToAdaptor(input, context)->utf8_adaptor.length();
 }
 
 // static
 const char* StringTraits<WTF::String>::GetData(const WTF::String& input,
                                                void* context) {
   return ToAdaptor(input, context)->utf8_adaptor.data();
 }
 
 // static
 bool StringTraits<WTF::String>::Read(StringDataView input,
                                      WTF::String* output) {
-  if (!input.is_null()) {
-    WTF::String result = WTF::String::fromUTF8(input.storage(), input.size());
-    output->swap(result);
-  } else if (!output->isNull()) {
-    WTF::String result;
-    output->swap(result);
-  }
+  WTF::String result = WTF::String::fromUTF8(input.storage(), input.size());
+  output->swap(result);
   return true;
 }
 
 }  // namespace mojo