Update HTTPHeaderMap wrappers to use AtomicString type for header values

Source/platform/network/HTTPRequest.h

Index: Source/platform/network/HTTPRequest.h
diff --git a/Source/platform/network/HTTPRequest.h b/Source/platform/network/HTTPRequest.h
index 7d80909e3018af03e67a783ad5fd7adcc6712e00..2f4b4272ac97e8370eb8724621b9d410f7556352 100644
--- a/Source/platform/network/HTTPRequest.h
+++ b/Source/platform/network/HTTPRequest.h
@@ -58,8 +58,8 @@ public:
     const Vector<unsigned char>& body() const { return m_body; }
 
     const HTTPHeaderMap& headerFields() const { return m_headerFields; }
-    void addHeaderField(const AtomicString& name, const String& value) { m_headerFields.add(name, value); }
-    void addHeaderField(const char* name, const String& value) { m_headerFields.add(name, value); }
+    void addHeaderField(const AtomicString& name, const AtomicString& value) { m_headerFields.add(name, value); }
+    void addHeaderField(const char* name, const AtomicString& value) { m_headerFields.add(name, value); }
 
 protected:
     HTTPRequest();