Use AtomicString type more consistently for mimeType / encoding

Source/core/loader/TextResourceDecoderBuilder.h

Index: Source/core/loader/TextResourceDecoderBuilder.h
diff --git a/Source/core/loader/TextResourceDecoderBuilder.h b/Source/core/loader/TextResourceDecoderBuilder.h
index c59221722a6ed1f840313a8beaa527d99ca714de..b6c27c7746400f8b48802f7653888c5e2d0c4ed0 100644
--- a/Source/core/loader/TextResourceDecoderBuilder.h
+++ b/Source/core/loader/TextResourceDecoderBuilder.h
@@ -42,13 +42,13 @@ 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();
@@ -57,8 +57,8 @@ private:
     PassOwnPtr<TextResourceDecoder> createDecoderInstance(Document*);
     void setupEncoding(TextResourceDecoder*, Document*);
 
-    String m_mimeType;
-    String m_encoding;
+    AtomicString m_mimeType;
+    AtomicString m_encoding;
     bool m_encodingWasChosenByUser;
 };