Replace many [TreatReturnedNullStringAs=Null] with nullable DOMString

Source/modules/serviceworkers/Headers.idl

Index: Source/modules/serviceworkers/Headers.idl
diff --git a/Source/modules/serviceworkers/Headers.idl b/Source/modules/serviceworkers/Headers.idl
index 8a2d3d66b63bbee19c2ef45991e250dd5e977f2a..1e612b170bb49c6157b2dd2425c2f3a361f47e18 100644
--- a/Source/modules/serviceworkers/Headers.idl
+++ b/Source/modules/serviceworkers/Headers.idl
@@ -13,7 +13,7 @@
 ] interface Headers {
     [RaisesException] void append(ByteString name, ByteString value);
     [ImplementedAs=remove, RaisesException] void delete(ByteString key);
-    [TreatReturnedNullStringAs=Null, RaisesException] ByteString? get(ByteString key);
+    [RaisesException] ByteString? get(ByteString key);
     [RaisesException] sequence<ByteString> getAll(ByteString name);
     [RaisesException] boolean has(ByteString key);
     [RaisesException] void set(ByteString key, ByteString value);