ensure_gn_version.py: reinstall if tool is missing

It's pretty easy to wedge things currently if you do:
  rm buildtools/linux64/gn

ensure_gn_version.py then gets stuck in a crashing loop where it
tries to run the tool with --version but it always fails w/ENOENT.
So improve the logic to redownload things when they're missing.

Change-Id: Id19c1fdbf9b1ed69930f5a6e3f192e1879ce8d62
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1697947
Auto-Submit: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676659}
1 file changed