Don't use is operatory to compare Python strings

`is` checks identity. `==` checks value. I can't think of a reason why
we would want the former in these scenarios.

More info:

* http://stackoverflow.com/a/1504742
* https://docs.python.org/2/reference/expressions.html#is

components/script/dom/bindings/codegen/GenerateCSS2PropertiesWebIDL.py

@@ -9,7 +9,7 @@ propList = eval(sys.stdin.read())
 props = ""
 for [prop, pref] in propList:
     extendedAttrs = ["Throws", "TreatNullAs=EmptyString"]
-    if pref is not "":
+    if pref != "":
         extendedAttrs.append("Pref=%s" % pref)
     if not prop.startswith("Moz"):
         prop = prop[0].lower() + prop[1:]