This adds support for set_global and get_global.

R=ahaas@chromium.org

Change-Id: I08bfa3c23080f473616970e9894cfb6e55a4f76d
Reviewed-on: https://chromium-review.googlesource.com/890744
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50963}