Move GN directory name assert into //BUILD.gn

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index ae90772b3fbc82eae2182778499e94081709d12d..cf1bc4c0d40fe7b2aec88b74085093df4b164285 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -26,6 +26,21 @@ declare_args() {
   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