Puts an allowed range on z-index.

Allows -1E6 to 1E6 as valid z-indices.  We reserve the values above
and below that for our own use.

[felix8a, 2012-03-09 23:27:58]

looks ok, but how about add a test for the client-side css sanitizer in
tests/src/com/google/caja/plugin/sanitizecss_test.js

[felix8a, 2012-03-21 00:15:35]

(verbal review).  other than removing es5mode restriction in the test, LGTM