Make depot_tools upload reviews to Gerrit by default

BUG=634946

Change-Id: I1724395d6ba223854e513543841bdbb317f3ede7
Reviewed-on: https://chromium-review.googlesource.com/410227Reviewed-by: Andrew Bonventre <andybons@chromium.org>
Commit-Queue: Aaron Gable <agable@chromium.org>

codereview.settings

 # This file is used by git cl to get repository specific information.
-CODE_REVIEW_SERVER: codereview.chromium.org
 CC_LIST: chromium-reviews@chromium.org
-VIEW_VC: https://chromium.googlesource.com/chromium/tools/depot_tools/+/
+CODE_REVIEW_SERVER: codereview.chromium.org
+GERRIT_HOST: True
 PROJECT: depot_tools
+VIEW_VC: https://chromium.googlesource.com/chromium/tools/depot_tools/+/