Don't set CHROME_ORIGIN environment variable

CL http://gerrit.chromium.org/gerrit/#change,10530 removes dependency of
chrome ebuild on CHROME_ORIGIN being set to GERRIT_SOURCE when user runs
cros_workon.  So remove this functionality.

BUG=chromium-os:21969
TEST=Ran cros_workon start/stop for chrome and cros-devutil

Change-Id: I04f7093b45f8e2871f369331bdafbba36443e21d
Reviewed-on: http://gerrit.chromium.org/gerrit/10531
Reviewed-by: David James <davidjames@chromium.org>
Commit-Ready: Ryan Cui <rcui@chromium.org>
Tested-by: Ryan Cui <rcui@chromium.org>