Use AtomicString type more consistently for mimeType / encoding

Source/core/loader/TextResourceDecoderBuilder.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 24 matching lines @@
 #include "wtf/PassRefPtr.h"
 #include "wtf/text/WTFString.h"
 
 namespace WebCore {
 
 class Document;
 class TextResourceDecoder;
 
 class TextResourceDecoderBuilder {
 public:
-    TextResourceDecoderBuilder(const String& mimeType, const String& encoding, bool encodingUserChoosen);
+    TextResourceDecoderBuilder(const AtomicString& mimeType, const AtomicString& encoding, bool encodingUserChoosen);
     ~TextResourceDecoderBuilder();
 
     PassOwnPtr<TextResourceDecoder> buildFor(Document*);
 
-    const String& mimeType() const { return m_mimeType; }
-    const String& encoding() const { return m_encoding; }
+    const AtomicString& mimeType() const { return m_mimeType; }
+    const AtomicString& encoding() const { return m_encoding; }
     bool encodingWasChosenByUser() const { return m_encodingWasChosenByUser; }
 
     void clear();
 
 private:
     PassOwnPtr<TextResourceDecoder> createDecoderInstance(Document*);
     void setupEncoding(TextResourceDecoder*, Document*);
 
-    String m_mimeType;
-    String m_encoding;
+    AtomicString m_mimeType;
+    AtomicString m_encoding;
     bool m_encodingWasChosenByUser;
 };
 
 } // namespace WebCore
 
 #endif // TextResourceDecoderBuilder_h