GN: gitignore //third_party/gn.

Depot tools download the GN here now.

Bug: 855791
Change-Id: Id2d60230331b90ff97018493892189deed5bbb77
Reviewed-on: https://chromium-review.googlesource.com/1118269Reviewed-by: agrieve <agrieve@chromium.org>
Commit-Queue: Greg Kraynov <kraynov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571102}

third_party/.gitignore

@@ -74,6 +74,7 @@
 /gles2_conform
 /glslang/src
 /glslang-angle/src
+/gn/
 /gnu_binutils/
 /google_appengine_cloudstorage
 /google_toolbox_for_mac/src