Duplicate property names can cause test to fail

Duplicate property names can cause test to fail
https://bugs.webkit.org/show_bug.cgi?id=119399

Reviewed by Dirk Pranke.

This problem was noticed when porting the patch for
http://wkb.ug/119357 to Blink: a test would fail because the converted
and test property lists weren't the same length, but they contained
all of the same property names. It looks like this is actually caused
by a bug in converting the HTML, which is being currently rewritten.
In the meantime, as long as this compares the number of unique
properties, it makes the test more robust.

* Scripts/webkitpy/w3c/test_converter_unittest.py:
(verify_prefixed_properties):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@153590 268f45cc-cd09-0410-ab3c-d52691b4dbfc