Get rid of useless calls to AtomicString::impl()

Source/core/loader/DocumentLoader.cpp

 /*
  * Copyright (C) 2006, 2007, 2008 Apple Inc. All rights reserved.
  * Copyright (C) 2011 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:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 551 matching lines @@
 void DocumentLoader::ensureWriter()
 {
     ensureWriter(m_response.mimeType());
 }
 
 void DocumentLoader::ensureWriter(const String& mimeType, const KURL& overridingURL)
 {
     if (m_writer)
         return;
 
-    String encoding = overrideEncoding().isNull() ? response().textEncodingName().impl() : overrideEncoding();
+    const String& encoding = overrideEncoding().isNull() ? response().textEncodingName().string() : overrideEncoding();
     m_writer = createWriterFor(m_frame, 0, requestURL(), mimeType, encoding, false, false);
     m_writer->setDocumentWasLoadedAsPartOfNavigation();
     // This should be set before receivedFirstData().
     if (!overridingURL.isEmpty())
         m_frame->document()->setBaseURLOverride(overridingURL);
 
     // Call receivedFirstData() exactly once per load.
     frameLoader()->receivedFirstData();
     m_frame->document()->maybeHandleHttpRefresh(m_response.httpHeaderField("Refresh"), Document::HttpRefreshFromHeader);
 }
@@ skipping 365 matching lines @@
 void DocumentLoader::replaceDocument(const String& source, Document* ownerDocument)
 {
     m_frame->loader().stopAllLoaders();
     m_writer = createWriterFor(m_frame, ownerDocument, m_frame->document()->url(), mimeType(), m_writer ? m_writer->encoding() : "",  m_writer ? m_writer->encodingWasChosenByUser() : false, true);
     if (!source.isNull())
         m_writer->appendReplacingData(source);
     endWriting(m_writer.get());
 }
 
 } // namespace WebCore