This CL updates use_sysroot to reflect which versions of the Jessie
sysroot are actually available.  It just changes the list from a
subtractive one to an additive one, and there should be no functional
changes.

R=dpranke@chromium.org
CC=sbc@chromium.org

Change-Id: I19497823bb968a37b24a97240a107fdbbdeeab37
Reviewed-on: https://chromium-review.googlesource.com/576855


Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487735}