This simplifies the process of bootstrapping gn standalone.

And libevent is not really used outside of base. base is actually its only client.

BUG=569352
TEST=See CL for details for how to reproduce this.
R=thestig@chromium.org
TBR=cpu@chromium.org

Review URL: https://codereview.chromium.org/1531573008

Cr-Commit-Position: refs/heads/master@{#366282}