Remove mojo::String.

content/child/mojo/type_converters.h

 // Copyright 2015 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 <stddef.h>
 #include <utility>
 
 #include "mojo/public/cpp/bindings/array.h"
-#include "third_party/WebKit/public/platform/WebString.h"
 #include "third_party/WebKit/public/platform/WebVector.h"
 
 namespace mojo {
 
 template <typename T, typename U>
 struct TypeConverter<Array<T>, blink::WebVector<U>> {
   static Array<T> Convert(const blink::WebVector<U>& vector) {
     Array<T> array(vector.size());
     for (size_t i = 0; i < vector.size(); ++i)
       array[i] = TypeConverter<T, U>::Convert(vector[i]);
     return std::move(array);
   }
 };
 
-template <>
-struct TypeConverter<String, blink::WebString> {
-  static String Convert(const blink::WebString& web_string) {
-    return String(web_string.utf8());
-  }
-};
-
 }  // namespace mojo