Update HTTPHeaderMap wrappers to use AtomicString type for header values

Source/platform/network/ResourceResponse.h

Index: Source/platform/network/ResourceResponse.h
diff --git a/Source/platform/network/ResourceResponse.h b/Source/platform/network/ResourceResponse.h
index 97fac4cf217c598fd74f9eb0b90cc2bf721e5822..6944da3a96daa8be7fe940fd2e7bee6d426a91a4 100644
--- a/Source/platform/network/ResourceResponse.h
+++ b/Source/platform/network/ResourceResponse.h
@@ -85,10 +85,10 @@ public:
     const AtomicString& httpStatusText() const;
     void setHTTPStatusText(const AtomicString&);
 
-    String httpHeaderField(const AtomicString& name) const;
-    String httpHeaderField(const char* name) const;
-    void setHTTPHeaderField(const AtomicString& name, const String& value);
-    void addHTTPHeaderField(const AtomicString& name, const String& value);
+    AtomicString httpHeaderField(const AtomicString& name) const;
+    const AtomicString& httpHeaderField(const char* name) const;
+    void setHTTPHeaderField(const AtomicString& name, const AtomicString& value);
+    void addHTTPHeaderField(const AtomicString& name, const AtomicString& value);
     void clearHTTPHeaderField(const AtomicString& name);
     const HTTPHeaderMap& httpHeaderFields() const;