XMLSerializer: Keep redundant but harmless default namespace declarations

Since crrev.com/632142, XMLSerializer has dropped redundant xmlns="..."
declarations. It matches to IE, Edge, and DOM P&S standard.  This CL
reverts the behaviro to unbreak existing applications.  The restored
behavior matches to Firefox and Safari.

* MarkupAccumulator::AppendElement():
  Even if ignore_namespace_definition_attribute_ is set, we drop a
  xmlns="..." only if its value is inconsistent with element's
  namespace.

* MarkupAccumulator::AppendStartTagOpen():
  if local_default_namespace is "" and ns is null, do not emit xmlns="".
  This avoids to serialize xmlns="" twice.

Bug: w3c/DOM-Parsing#47
Bug: 940204
Change-Id: I2978ddc9a3f9511d227a9a1b902f1811ac1c3c07

Author: tkent-google

domparsing/XMLSerializer-serializeToString.html

@@ -45,6 +45,13 @@ <h1>domparsing_XMLSerializer_serializeToString</h1>
   assert_equals(serialize(root), '<root xmlns="urn:bar"><outer xmlns=""><inner>value1</inner></outer></root>');
 }, 'Check if there is no redundant empty namespace declaration.');
 
+test(function() {
+  assert_equals(serialize(parse('<root xmlns=""><child xmlns=""/></root>')),
+                '<root><child/></root>');
+  assert_equals(serialize(parse('<root xmlns="u1"><child xmlns="u1"/></root>')),
+                '<root xmlns="u1"><child/></root>');
+}, 'Check if redundant xmlns="..." is dropped.');
+
 test(function() {
   const root = parse('<root xmlns="uri1"/>');
   const child = root.ownerDocument.createElement('child');
@@ -145,8 +152,6 @@ <h1>domparsing_XMLSerializer_serializeToString</h1>
 
 test(function() {
   assert_equals(serialize(parse('<root><child/></root>')), '<root><child/></root>');
-  assert_equals(serialize(parse('<root><child xmlns=""/></root>')), '<root><child/></root>');
-  assert_equals(serialize(parse('<root xmlns="u1"><child xmlns="u1"/></root>')), '<root xmlns="u1"><child/></root>');
   assert_equals(serialize(parse('<root xmlns="u1"><p:child xmlns:p="u1"/></root>')), '<root xmlns="u1"><child xmlns:p="u1"/></root>');
 }, 'Check if start tag serialization drops element prefix if the namespace is same as inherited default namespace.');