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

Source/core/html/parser/HTMLSrcsetParser.h

Index: Source/core/html/parser/HTMLSrcsetParser.h
diff --git a/Source/core/html/parser/HTMLSrcsetParser.h b/Source/core/html/parser/HTMLSrcsetParser.h
index 8964ffbcd5f61ee309ab67d2e4fe83855e89c736..861df34a418c7e0a15efb08a88c96c0bfdb8c6d2 100644
--- a/Source/core/html/parser/HTMLSrcsetParser.h
+++ b/Source/core/html/parser/HTMLSrcsetParser.h
@@ -50,7 +50,12 @@ public:
 
     String toString() const
     {
-        return m_string.toString();
+        return String(m_string.toString());
+    }
+
+    AtomicString toAtomicString() const
+    {
+        return AtomicString(m_string.toString());
     }
 
     inline float scaleFactor() const