And tweak --depot-tools to go first on PATH

Change-Id: Iee53d84fd028ac0c2de6f872184cbce51e84c54f
Reviewed-on: https://chromium-review.googlesource.com/1028210Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52788}