Add a "ensure_bootstrap" script, which syncs the bootstrapped state of
"depot_tools" so that remaining operations become read-only.

BUG=chromium:769369
TEST=local

Change-Id: I2dab36112b378a926d8201d2ab31a7daa9f5da29
Reviewed-on: https://chromium-review.googlesource.com/691318
Commit-Queue: Daniel Jacques <dnj@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>