Add a GN assert that output dir doesn't have a platform name

build/config/BUILD.gn

Index: build/config/BUILD.gn
diff --git a/build/config/BUILD.gn b/build/config/BUILD.gn
index abd6c2f09d97144ac1b22973600d040ba3392238..5197304a6eeb4f63e7241bd540632f105b12208f 100644
--- a/build/config/BUILD.gn
+++ b/build/config/BUILD.gn
@@ -27,6 +27,21 @@ declare_args() {
   dcheck_always_on = false
 }
 
+# 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") {
+  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
+}
+
 # TODO(brettw) Most of these should be removed. Instead of global feature
 # flags, we should have more modular flags that apply only to a target and its
 # dependents. For example, depending on the "x11" meta-target should define