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>
diff --git a/.gitignore b/.gitignore
index 9d489d6..46a48b7 100644
--- a/.gitignore
+++ b/.gitignore
@@ -38,3 +38,5 @@
 buildtools
 tools/clang
 third_party/llvm-build
+bin/gn
+bin/gn.exe