Serialize prefixes for HTML fragments correctly

Serialize prefixes for HTML fragments correctly as described in the spec:
https://html.spec.whatwg.org/#serialising-html-fragments

Bug: 651762
TEST=external/wpt/html/syntax/serializing-html-fragments/serializing.html

Change-Id: I577841f60eea118ce0039e53f95d970c20566261
Reviewed-on: https://chromium-review.googlesource.com/883862Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Rob Buis <rob.buis@samsung.com>
Cr-Commit-Position: refs/heads/master@{#532068}

third_party/WebKit/LayoutTests/external/wpt/html/syntax/serializing-html-fragments/serializing-expected.txt

 This is a testharness.js-based test.
-Found 236 tests; 224 PASS, 12 FAIL, 0 TIMEOUT, 0 NOTRUN.
+Found 236 tests; 228 PASS, 8 FAIL, 0 TIMEOUT, 0 NOTRUN.
 PASS innerHTML 0 
 PASS innerHTML 1 <a></a>
 PASS innerHTML 2 <a b="c"></a>
@@ -9,7 +9,7 @@ PASS innerHTML 5 <a b="&nbsp;"></a>
 PASS innerHTML 6 <a b="&quot;"></a>
 PASS innerHTML 7 <a b="<"></a>
 PASS innerHTML 8 <a b=">"></a>
-FAIL innerHTML 9 <svg xlink:href="a"></svg> assert_equals: expected "<svg xlink:href=\"a\"></svg>" but got "<svg xmlns:xlink=\"http://www.w3.org/1999/xlink\" xlink:href=\"a\"></svg>"
+PASS innerHTML 9 <svg xlink:href="a"></svg>
 PASS innerHTML 10 <svg xmlns:svg="test"></svg>
 PASS innerHTML 11 a
 PASS innerHTML 12 &amp;
@@ -37,7 +37,7 @@ PASS outerHTML 5 <span><a b="&nbsp;"></a></span>
 PASS outerHTML 6 <span><a b="&quot;"></a></span>
 PASS outerHTML 7 <span><a b="<"></a></span>
 PASS outerHTML 8 <span><a b=">"></a></span>
-FAIL outerHTML 9 <span><svg xlink:href="a"></svg></span> assert_equals: expected "<span><svg xlink:href=\"a\"></svg></span>" but got "<span><svg xmlns:xlink=\"http://www.w3.org/1999/xlink\" xlink:href=\"a\"></svg></span>"
+PASS outerHTML 9 <span><svg xlink:href="a"></svg></span>
 PASS outerHTML 10 <span><svg xmlns:svg="test"></svg></span>
 PASS outerHTML 11 <span>a</span>
 PASS outerHTML 12 <span>&amp;</span>
@@ -57,13 +57,13 @@ PASS outerHTML 25 <span><!--data--></span>
 PASS outerHTML 26 <span><a><b><c></c></b><d>e</d><f><g>h</g></f></a></span>
 PASS outerHTML 27 <span b="c"></span>
 PASS innerHTML Attribute in the XML namespace
-FAIL innerHTML Attribute in the XML namespace with the prefix not set to xml: assert_equals: expected "<svg xml:foo=\"test\"></svg>" but got "<svg abc:foo=\"test\"></svg>"
+PASS innerHTML Attribute in the XML namespace with the prefix not set to xml:
 PASS innerHTML Non-'xmlns' attribute in the xmlns namespace
 PASS innerHTML 'xmlns' attribute in the xmlns namespace
 PASS innerHTML Attribute in non-standard namespace
 PASS innerHTML <span> starting with U+000A
 PASS outerHTML Attribute in the XML namespace
-FAIL outerHTML Attribute in the XML namespace with the prefix not set to xml: assert_equals: expected "<span><svg xml:foo=\"test\"></svg></span>" but got "<span><svg abc:foo=\"test\"></svg></span>"
+PASS outerHTML Attribute in the XML namespace with the prefix not set to xml:
 PASS outerHTML Non-'xmlns' attribute in the xmlns namespace
 PASS outerHTML 'xmlns' attribute in the xmlns namespace
 PASS outerHTML Attribute in non-standard namespace

third_party/WebKit/Source/core/editing/serializers/MarkupFormatter.cpp

@@ -347,13 +347,6 @@ void MarkupFormatter::AppendCloseTag(StringBuilder& result,
   result.Append('>');
 }
 
-static inline bool AttributeIsInSerializedNamespace(
-    const Attribute& attribute) {
-  return attribute.NamespaceURI() == XMLNames::xmlNamespaceURI ||
-         attribute.NamespaceURI() == XLinkNames::xlinkNamespaceURI ||
-         attribute.NamespaceURI() == XMLNSNames::xmlnsNamespaceURI;
-}
-
 void MarkupFormatter::AppendAttribute(StringBuilder& result,
                                       const Element& element,
                                       const Attribute& attribute,
@@ -361,7 +354,15 @@ void MarkupFormatter::AppendAttribute(StringBuilder& result,
   bool document_is_html = SerializeAsHTMLDocument(element);
 
   QualifiedName prefixed_name = attribute.GetName();
-  if (document_is_html && !AttributeIsInSerializedNamespace(attribute)) {
+  if (document_is_html) {
+    if (attribute.NamespaceURI() == XMLNSNames::xmlnsNamespaceURI) {
+      if (!attribute.Prefix() && attribute.LocalName() != g_xmlns_atom)
+        prefixed_name.SetPrefix(g_xmlns_atom);
+    } else if (attribute.NamespaceURI() == XMLNames::xmlNamespaceURI) {
+      prefixed_name.SetPrefix(g_xml_atom);
+    } else if (attribute.NamespaceURI() == XLinkNames::xlinkNamespaceURI) {
+      prefixed_name.SetPrefix(g_xlink_atom);
+    }
     result.Append(' ');
     result.Append(prefixed_name.ToString());
   } else {