Move GN directory name assert into //BUILD.gn

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.
 
@@ skipping 855 matching lines @@
       ]
 
       if (target_cpu == "x86") {
         deps += [ "//chrome/tools/crash_service:crash_service_win64" ]
       }
     } else {
       deps += [ "//breakpad:minidump_stackwalk($host_toolchain)" ]
     }
   }
 }
+
+# Because of the source assignment filter, many targets end up over-filtering
+# their sources if the output directory contains a platform name. Assert that
+# this doesn't happen. http://crbug.com/548283
+template("assert_valid_out_dir") {
+  # List copied from //build/config/BUILDCONFIG.gn.
+  set_sources_assignment_filter([
+                                  "*\bandroid/*",
+                                  "*\bchromeos/*",
+                                  "*\bcocoa/*",
+                                  "*\bios/*",
+                                  "*\blinux/*",
+                                  "*\bmac/*",
+                                  "*\bposix/*",
+                                  "*\bwin/*",
+                                ])
+  assert(target_name != "")  # Mark as used.
+  sources = invoker.actual_sources
+  assert(
+      sources == invoker.actual_sources,
+      "Do not use a platform name in your output directory (found \"$root_build_dir\"). http://crbug.com/548283")
+}
+
+assert_valid_out_dir("_unused") {
+  actual_sources = [ "$root_build_dir/foo" ]
+}