Add warnings if CHROME_ORIGIN is set on cros_workon start chromeos-chrome
Add warnings if CHROME_ORIGIN is set on cros_workon start chromeos-chrome

Because the chromeos-chrome ebuild will no longer force GERRIT_SOURCE,
warn a user that is starting to workon chromeos-chrome that they
may not be getting the experience they expect.

BUG=None
TEST=Ran cros_workon with CHROME_ORIGIN unset, set to GERRIT_SOURCE
  and set to LOCAL_SOURCE
  Saw warning only on LOCAL_SOURCE
  Ran cros_workon start kernel with CHROME_ORIGIN set and saw
    no warnings

Change-Id: Iecbc793d3b190a12a126d42a1bfaee6d8846e9ec
Reviewed-on: https://gerrit.chromium.org/gerrit/10815
Tested-by: Jon Kliegman <7addd549d0b04b2dc663a0a7eb2c1810caf11d96@chromium.org>
Reviewed-by: Ryan Cui <5ce1592d53754a87a44fc2eb7b63d5f77cf50fdb@chromium.org>
Commit-Ready: Jon Kliegman <7addd549d0b04b2dc663a0a7eb2c1810caf11d96@chromium.org>

marineam/coreos-dev-util,mgireesh05/dev-util,mgireesh05/dev-util,coreos/dev-util,marineam/coreos-dev-util,coreos/dev-util,coreos/dev-util,mgireesh05/dev-util