On buildbot (only), we don't trust PYTHONPATH, so we have set
VPYTHON_CLEAR_PYTHONPATH to make the first invocation of vpython in a
given chain of python invocations clear PYTHONPATH.

This has the unfortunate side-effect that python scripts that set
PYTHONPATH and then call into vpython (like presubmit) end up
having their PYTHONPATH modifications removed.

As a workaround (until I clean up PYTHONPATH in buildbot for real), fix
presubmit_canned_checks so that it clears VPYTHON_CLEAR_PYTHONPATH when it
adjusts PYTHONPATH.

R=charlie@chromium.org, nodir@chromium.org, vadimsh@chromium.org

Bug: 825290,825174
Change-Id: Ib5f73add1726fdf3c335d26fc0af76cfe3b747b2
Reviewed-on: https://chromium-review.googlesource.com/978632Reviewed-by: Charlie Andrews <charliea@chromium.org>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>