Pull gn binary

tools/gn/bin/README.txt

Index: tools/gn/bin/README.txt
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+This folder contains GN binaries. They should be automatically downloaded from
+Google Storage by gclient runhooks for the current platform.
+
+
+To upload a file:
+  python ~/depot_tools/upload_to_google_storage.py -b chromium-gn <FILENAME>
+
+To download a file given a .sha1 file:
+  python ~/depot_tools/download_from_google_storage.py -b chromium-gn -s <FILENAME>.sha1
+
+List the contents of GN's Google Storage bucket:
+  python ~/depot_tools/third_party/gsutil/gsutil ls gs://chromium-gn/
+
+To initialize gsutil's credentials:
+  python ~/depot_tools/third_party/gsutil/gsutil config
+
+  That will give a URL which you should log into with your web browser. The
+  username should be the one that is on the ACL for the "chromium-gn" bucket
+  (probably your @google.com address). Contact the build team for help getting
+  access if necessary.
+
+  Copy the code back to the command line util. Ignore the project ID (it's OK
+  to just leave blank when prompted).
+
+gsutil documentation:
+  https://developers.google.com/storage/docs/gsutil