Utilize Python context managers for opening/closing files

In some of these cases, files were not being closed

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

@@ -18,9 +18,8 @@ for [prop, pref] in propList:
     props += "  [%s] attribute DOMString %s;\n" % (", ".join(extendedAttrs),
                                                    prop)
 
-idlFile = open(sys.argv[1], "r")
-idlTemplate = idlFile.read()
-idlFile.close()
+with open(sys.argv[1], "r") as idlFile:
+    idlTemplate = idlFile.read()
 
 print ("/* THIS IS AN AUTOGENERATED FILE.  DO NOT EDIT */\n\n" +
        string.Template(idlTemplate).substitute({"props": props}))