gn: fix bootstrap script Review-Url: https://codereview.chromium.org/2446643002 Cr-Original-Commit-Position: refs/heads/master@{#427094} Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src Cr-Mirrored-Commit: 03c1d5ac093a45294eb59714db7f158de4631f54