Add defines to the "gn help" index.

R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/494493002

git-svn-id: http://src.chromium.org/svn/trunk/src/tools/gn@290852 4ff67af0-8c30-449e-8e8b-ad334ec8d88c
GN: fix bootstrap.py due to recent changes in //base

NOTRY=true
R=brettw@chromium.org
BUG=

Review URL: https://codereview.chromium.org/491123002

git-svn-id: http://src.chromium.org/svn/trunk/src/tools/gn@290931 4ff67af0-8c30-449e-8e8b-ad334ec8d88c