chromium: Update beta and dev release channels.

The following new versions were introduced:

beta: 28.0.1500.45 - builds fine and tested
dev:  29.0.1521.3  - builds fine and tested

Although the version from the dev release channel isn't the latest found on
omahaproxy but it's the latest one, that actually has tarballs available.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>