Flip Linux ChromeOS trybot to GN

This flips linux_chromium_chromeos_compile_rel_ng.

Review URL: https://codereview.chromium.org/1867983002

Cr-Commit-Position: refs/heads/master@{#387423}

diff --git a/BUILD.gn b/BUILD.gn
index 3172eb8..c6d0396 100644
--- a/BUILD.gn
+++ b/BUILD.gn

@@ -876,6 +876,15 @@
   }
 }
 
+# For compatibility with GYP. The linux_chromium_chromeos_rel_ng and
+# linux_chromium_chromeos_compile_rel_ng bots reference this target as
+# something to build, but all targets for those bots to compile are set
+# up differently.
+# TODO bug 601920: Remove reference to aura_builder on bot config and delete
+# this group.
+group("aura_builder") {
+}
+
 # Because of the source assignment filter, many targets end up over-filtering
 # their sources if the output directory contains a platform name. Assert that
 # this doesn't happen. http://crbug.com/548283