Mark the root GN build target as test only.

BUILD.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # This is the root build file for GN. GN will start processing by loading this
 # file, and recursively load all dependencies until all dependencies are either
 # resolved or known not to exist (which will cause the build to fail). So if
 # you add a new build file, there must be some path of dependencies from this
 # file to your new one or GN won't know about it.
 
 import("//build/config/ui.gni")
 
 # In GN, a "group" is a dummy target that just lists other targets.
 group("root") {
+  # This should not be linked into production and depends on test targets.
+  testonly = true
+
   # Note that some dependencies are commented out. These are things that are
   # currently written but not hooked up to the build yet. They may need to be
   # completed or possibly just tested and then re-enabled.
   deps = [
     "//apps",
     "//ash",
     "//cc",
     "//cc/blink",
     "//chrome/browser",
     "//chrome/browser/devtools",
@@ skipping 206 matching lines @@
     ]
   }
 
   if (use_ozone) {
     deps += [
       "//ui/ozone",
       "//ui/ozone/demo",
     ]
   }
 }