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 <[email protected]>
Commit-Queue: Daniel Bratell <[email protected]>
Cr-Commit-Position: refs/heads/master@{#593217}
diff --git a/docs/emacs.md b/docs/emacs.md
index 1dc555a..434d3252 100644
--- a/docs/emacs.md
+++ b/docs/emacs.md
@@ -14,7 +14,6 @@
 
     (load-file "src/tools/emacs/flymake-chromium.el")
 
-
 ## [ycmd](https://github.com/Valloric/ycmd) (YouCompleteMe) + flycheck
 
 [emacs-ycmd](https://github.com/abingham/emacs-ycmd) in combination with
@@ -163,6 +162,13 @@
 
 key to format the current line (even a long line) or region.
 
+## BUILD.gn and .gni files
+
+BUILD.gn and gni files are supported by the official gn-mode which can
+be enabled with:
+
+    (load-file "src/tools/emacs/gn.el")
+
 ## Highlight long lines
 
 One nice way to highlight long lines and other style issues: