git_cache.py: Don't bootstrap from Google storage for unrecognized repos.
git_cache.py: Don't bootstrap from Google storage for unrecognized repos.

Also - Turn on GC for the unrecognized repos.

BUG=skia:6241
TEST=local
  git_cache.py populate with chromium and non-chromium repo.

Change-Id: I100d3665e317b29c5b3f69b49c165ce88cd019d6
Reviewed-on: https://chromium-review.googlesource.com/455979
Commit-Queue: Ryan Tseng <2190fe9a39b4920ec7bd59c848ec5cccd163056f@chromium.org>
Reviewed-by: Aaron Gable <bbed39beedae4cdb499af742d2fcd6c63934d93b@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools