Make calls to AtomicString(const String&) explicit in web/

Source/core/dom/Document.h

Index: Source/core/dom/Document.h
diff --git a/Source/core/dom/Document.h b/Source/core/dom/Document.h
index 475970ebc3c6bed3d3e3729130a2575189605258..d8227ebc310d466723c797dfe37cb8d49a5ffb00 100644
--- a/Source/core/dom/Document.h
+++ b/Source/core/dom/Document.h
@@ -538,7 +538,7 @@ public:
     void setBaseURLOverride(const KURL&);
     const KURL& baseURLOverride() const { return m_baseURLOverride; }
     const KURL& baseElementURL() const { return m_baseElementURL; }
-    const String& baseTarget() const { return m_baseTarget; }
+    const AtomicString& baseTarget() const { return m_baseTarget; }
     void processBaseElement();
 
     KURL completeURL(const String&) const;
@@ -1132,7 +1132,7 @@ private:
     // takes precedence.
     String m_documentURI;
 
-    String m_baseTarget;
+    AtomicString m_baseTarget;
 
     RefPtr<DocumentType> m_docType;
     OwnPtr<DOMImplementation> m_implementation;