Update HTTPHeaderMap wrappers to use AtomicString type for header values

Source/platform/network/HTTPHeaderMap.h

Index: Source/platform/network/HTTPHeaderMap.h
diff --git a/Source/platform/network/HTTPHeaderMap.h b/Source/platform/network/HTTPHeaderMap.h
index 6c4be87407a71d5f928ed848a09a87a785245015..8b3be52c8d37e58f5dec745b90b9ec8fe4b94eaa 100644
--- a/Source/platform/network/HTTPHeaderMap.h
+++ b/Source/platform/network/HTTPHeaderMap.h
@@ -57,7 +57,7 @@ public:
 
     // Alternate accessors that are faster than converting the char* to AtomicString first.
     bool contains(const char*) const;
-    AtomicString get(const char*) const;
+    const AtomicString& get(const char*) const;
     AddResult add(const char* name, const AtomicString& value);
 
 };