Move //tools/gs.py into //mojo/public/tools

This move enables download_network_service.py to cleanly use this function.
Since gs.py is now used from the SDK, it takes in the path to find_depot_tools
in order to import it. download_network_service.py correspondingly takes in
that path, similarly to download_shell_binary.py.

R=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/870143008
diff --git a/DEPS b/DEPS
index 16ced94..a12af1d 100644
--- a/DEPS
+++ b/DEPS
@@ -322,7 +322,10 @@
   {
     'name': 'download_network_service',
     'pattern': '',
-    'action': [ 'python', 'src/mojo/public/tools/download_network_service.py' ],
+    'action': [ 'python',
+                'src/mojo/public/tools/download_network_service.py',
+                '--tools-directory', '../../../tools',
+    ],
   },
   {
     # Ensure that we don't accidentally reference any .pyc files whose