W3C test import script prefixes some properties that it shouldn't

Some prefixed properties also have unprefixed implemetations, but when
the implementations aren't identical, they aren't declared in
CSSPropertyNames.in on the same line. This fixes the import script so
that it catches all instances of prefixed properties that have an
unprefixed variant, and does not add the prefix in those cases.

Port from WebKit of http://wkb.ug/119357

BUG=266595

Review URL: https://chromiumcodereview.appspot.com/21429004

git-svn-id: svn://svn.chromium.org/blink/trunk@155292 bbb929c8-8fbe-4397-9dbb-9b2b20218538