Use AtomicString type more consistently for mimeType / encoding

Source/core/loader/DocumentWriter.h

Index: Source/core/loader/DocumentWriter.h
diff --git a/Source/core/loader/DocumentWriter.h b/Source/core/loader/DocumentWriter.h
index 0f7eb5b1a1aa5d13e2213f7116c598b6de131d8f..4e79045a1e909594daba1502b26a84ff9a21bc72 100644
--- a/Source/core/loader/DocumentWriter.h
+++ b/Source/core/loader/DocumentWriter.h
@@ -45,7 +45,7 @@ class TextResourceDecoder;
 class DocumentWriter : public RefCounted<DocumentWriter> {
     WTF_MAKE_NONCOPYABLE(DocumentWriter);
 public:
-    static PassRefPtr<DocumentWriter> create(Document*, const String& mimeType = "", const String& encoding = "", bool encodingUserChoosen = false);
+    static PassRefPtr<DocumentWriter> create(Document*, const AtomicString& mimeType = emptyAtom, const AtomicString& encoding = emptyAtom, bool encodingUserChoosen = false);
 
     ~DocumentWriter();
 
@@ -57,8 +57,8 @@ public:
 
     void addData(const char* bytes, size_t length);
 
-    const String& mimeType() const { return m_decoderBuilder.mimeType(); }
-    const String& encoding() const { return m_decoderBuilder.encoding(); }
+    const AtomicString& mimeType() const { return m_decoderBuilder.mimeType(); }
+    const AtomicString& encoding() const { return m_decoderBuilder.encoding(); }
     bool encodingWasChosenByUser() const { return m_decoderBuilder.encodingWasChosenByUser(); }
 
     // Exposed for DocumentLoader::replaceDocument.
@@ -69,7 +69,7 @@ public:
     void setDocumentWasLoadedAsPartOfNavigation();
 
 private:
-    DocumentWriter(Document*, const String& mimeType, const String& encoding, bool encodingUserChoosen);
+    DocumentWriter(Document*, const AtomicString& mimeType, const AtomicString& encoding, bool encodingUserChoosen);
 
     PassRefPtr<Document> createDocument(const KURL&);