gn: Support build directories outside the source tree.

tools/gn/action_target_generator.cc

Index: tools/gn/action_target_generator.cc
diff --git a/tools/gn/action_target_generator.cc b/tools/gn/action_target_generator.cc
index 793635937b9ea15576fcffe069607ad6f41f1f19..09c89e4ca949b5c36e11b883ef665a6b5297351c 100644
--- a/tools/gn/action_target_generator.cc
+++ b/tools/gn/action_target_generator.cc
@@ -73,7 +73,8 @@ bool ActionTargetGenerator::FillScript() {
     return false;
 
   SourceFile script_file =
-      scope_->GetSourceDir().ResolveRelativeFile(value->string_value());
+      scope_->GetSourceDir().ResolveRelativeFile(value->string_value(),
+          scope_->settings()->build_settings()->root_path_utf8());
   if (script_file.value().empty()) {
     *err_ = Err(*value, "script name is empty");
     return false;