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.
 
 import("//build/config/features.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/ui.gni")
 import("//build_overrides/v8.gni")
 import("//media/media_options.gni")
 import("//third_party/openh264/openh264_args.gni")
 
 if (is_android) {
   import("//build/config/android/config.gni")
 }
 
 declare_args() {
   # A list of extra dependencies to add to the root target. This allows a
   # checkout to add additional targets without explicitly changing any checked-
   # in files.
   root_extra_deps = []
 }
 
+# Because of the source assignment filter, many targets end up over-filtering

[brettw, 2016/02/25 22:30:13]

Can you put this at the bottom so the file is more clear just reading from
top-to-bottom?

No other file will get loaded until this file is completed, so it won't hurt
your use-case, and we shouldn't have issues with targets in this file.

[agrieve, 2016/02/26 21:14:41]

Moved to the bottom, but found out that setting an explicit source filter is
required on linux (so I added it). Without it, I'm seeing:

ERROR at //ppapi/native_client/BUILD.gn:48:3: Empty sources for copy command.
  copy("nacl_irt_debug") {
  ^-----------------------
You have to specify at least one file to copy in the "sources".

+# 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") {
+  assert(target_name != "")  # Mark as used.
+  assert(
+      invoker.sources == invoker.actual_sources,
+      "Do not use a platform name in your output directory (found \"$root_build_dir\"). http://crbug.com/548283")
+}
+
+assert_valid_out_dir("_check_output_dir_has_no_platform_name") {
+  actual_sources = [ "$root_build_dir/foo" ]
+  sources = actual_sources
+}
+
 # This file defines the following five main targets:
 #
 # "both_gn_and_gyp" should list every root target (target that nothing else
 # depends on) built by GN that is also built in the GYP build.
 #
 # "gn_all" should (transitively) cause everything to be built; if you run
 # 'ninja gn_all' and then 'ninja all', the second build should do no work.
 #
 # "gn_only" should list every root target that is *not* intended to be built in
 # a GYP build. Because GN has different rules for deciding what an 'all' build
@@ skipping 826 matching lines @@
       ]
 
       if (target_cpu == "x86") {
         deps += [ "//chrome/tools/crash_service:crash_service_win64" ]
       }
     } else {
       deps += [ "//breakpad:minidump_stackwalk($host_toolchain)" ]
     }
   }
 }