XMLSerializer: Fix prefix generation algorithm

Fix it in order to match to DOM Parsing and Serialization specification.
We should not expose hash values of strings.

Bug: 928639
Change-Id: I589572d87715805961f2931c6808c1bd38acade6
Reviewed-on: https://chromium-review.googlesource.com/c/1454070
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Auto-Submit: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629490}

domparsing/XMLSerializer-serializeToString.html

@@ -55,6 +55,16 @@ <h1>domparsing_XMLSerializer_serializeToString</h1>
     '<root attr="&#xD;"/>', '<root attr="&#13;"/>']);
 }, 'check XMLSerializer.serializeToString escapes attribute values for roundtripping');
 
+test(function() {
+  const input = '<root><child1/><child2/></root>';
+  const root = (new DOMParser()).parseFromString(input, 'text/xml').documentElement;
+  root.firstChild.setAttributeNS('uri1', 'attr1', 'value1');
+  root.firstChild.setAttributeNS('uri2', 'attr2', 'value2');
+  root.lastChild.setAttributeNS('uri3', 'attr3', 'value3');
+  const xmlString = (new XMLSerializer()).serializeToString(root);
+  assert_equals(xmlString, '<root><child1 xmlns:ns1="uri1" ns1:attr1="value1" xmlns:ns2="uri2" ns2:attr2="value2"/><child2 xmlns:ns3="uri3" ns3:attr3="value3"/></root>');
+}, 'Check if generated prefixes match to "ns${index}".');
+
 </script>
  </body>
 </html>