Use AtomicString type more consistently for mimeType / encoding

Source/platform/mhtml/ArchiveResource.h

Index: Source/platform/mhtml/ArchiveResource.h
diff --git a/Source/platform/mhtml/ArchiveResource.h b/Source/platform/mhtml/ArchiveResource.h
index fdf0e55b927e41bdf1f33d8dceb678bfa377bdde..3bfde70511e2321cdab93a5b9fae687a2d660018 100644
--- a/Source/platform/mhtml/ArchiveResource.h
+++ b/Source/platform/mhtml/ArchiveResource.h
@@ -41,24 +41,24 @@ class PLATFORM_EXPORT ArchiveResource : public RefCounted<ArchiveResource> {
 public:
     static PassRefPtr<ArchiveResource> create(PassRefPtr<SharedBuffer>, const KURL&, const ResourceResponse&);
     static PassRefPtr<ArchiveResource> create(PassRefPtr<SharedBuffer>, const KURL&,
-        const String& mimeType, const String& textEncoding, const String& frameName,
+        const AtomicString& mimeType, const AtomicString& textEncoding, const String& frameName,
         const ResourceResponse& = ResourceResponse());
 
     const KURL& url() const { return m_url; }
     const ResourceResponse& response() const { return m_response; }
     SharedBuffer* data() const { return m_data.get(); }
-    const String& mimeType() const { return m_mimeType; }
-    const String& textEncoding() const { return m_textEncoding; }
+    const AtomicString& mimeType() const { return m_mimeType; }
+    const AtomicString& textEncoding() const { return m_textEncoding; }
     const String& frameName() const { return m_frameName; }
 
 private:
-    ArchiveResource(PassRefPtr<SharedBuffer>, const KURL&, const String& mimeType, const String& textEncoding, const String& frameName, const ResourceResponse&);
+    ArchiveResource(PassRefPtr<SharedBuffer>, const KURL&, const AtomicString& mimeType, const AtomicString& textEncoding, const String& frameName, const ResourceResponse&);
 
     KURL m_url;
     ResourceResponse m_response;
     RefPtr<SharedBuffer> m_data;
-    String m_mimeType;
-    String m_textEncoding;
+    AtomicString m_mimeType;
+    AtomicString m_textEncoding;
     String m_frameName;
 };