fix gn bootstrap once again
fix gn bootstrap once again

BUG=None
TEST=./tools/gn/bootstrap/bootstrap.py
TBR=brettw@chromium.org

Review URL: https://codereview.chromium.org/1455743002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#360131}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk