Add html5lib to the list of modules in webkitpy/thirdparty/wpt/wpt/tools.

third_party/WebKit/Tools/Scripts/webkitpy/thirdparty/wpt/wpt/tools/html5lib/html5lib/serializer/__init__.py

Index: third_party/WebKit/Tools/Scripts/webkitpy/thirdparty/wpt/wpt/tools/html5lib/html5lib/serializer/__init__.py
diff --git a/third_party/WebKit/Tools/Scripts/webkitpy/thirdparty/wpt/wpt/tools/html5lib/html5lib/serializer/__init__.py b/third_party/WebKit/Tools/Scripts/webkitpy/thirdparty/wpt/wpt/tools/html5lib/html5lib/serializer/__init__.py
new file mode 100644
index 0000000000000000000000000000000000000000..8380839a6d3e130e7ddd0a1e59f4d046fa824e4d
--- /dev/null
+++ b/third_party/WebKit/Tools/Scripts/webkitpy/thirdparty/wpt/wpt/tools/html5lib/html5lib/serializer/__init__.py
@@ -0,0 +1,16 @@
+from __future__ import absolute_import, division, unicode_literals
+
+from .. import treewalkers
+
+from .htmlserializer import HTMLSerializer
+
+
+def serialize(input, tree="etree", format="html", encoding=None,
+              **serializer_opts):
+    # XXX: Should we cache this?
+    walker = treewalkers.getTreeWalker(tree)
+    if format == "html":
+        s = HTMLSerializer(**serializer_opts)
+    else:
+        raise ValueError("type must be html")
+    return s.render(walker(input), encoding)