Mojo C++ bindings: switch WebKit mojom targets to use STL/WTF types.

content/browser/mime_registry_impl.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 CONTENT_BROWSER_MIME_REGISTRY_IMPL_H_
 #define CONTENT_BROWSER_MIME_REGISTRY_IMPL_H_
 
 #include "third_party/WebKit/public/platform/mime_registry.mojom.h"
 
 namespace content {
 
 class MimeRegistryImpl : public blink::mojom::MimeRegistry {
  public:
   MimeRegistryImpl();
   ~MimeRegistryImpl() override;
 
   static void Create(blink::mojom::MimeRegistryRequest request);
 
  private:
   void GetMimeTypeFromExtension(
-      const mojo::String& extension,
+      const std::string& extension,
       const GetMimeTypeFromExtensionCallback& callback) override;
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_MIME_REGISTRY_IMPL_H_