drop depot_tools dependency in bin/fetch-gn

... and copy gn to bin/ when done to make it easy to find.

Change-Id: I1ec405b4c45efb828626ff7d904a417f69b39cb2
Reviewed-on: https://skia-review.googlesource.com/6962
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
2 files changed