Bring back gn.el for the Emacs GN mode

gn.el used to be in the chromium source tree until gn moved
elsewhere. Now it lives somewhere else,
https://gn.googlesource.com/gn/+/master/tools/gn/misc/emacs/gn-mode.el
which makes it less useful.

This is a copy of that file as of today. The file is not updated
often, and when it is, it's just to add more keywords so
nothing critical.

Bug: 887937
Change-Id: I39335815a2cc8ecfe2b0e1728443f6abe84a85fa
Reviewed-on: https://chromium-review.googlesource.com/1238462
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#593217}
2 files changed