gn: Support build directories outside the source tree.

tools/gn/functions.cc

Index: tools/gn/functions.cc
diff --git a/tools/gn/functions.cc b/tools/gn/functions.cc
index c68eda53adcae947ee252754d8c5c4a2b416e917..b746bcdf78193d348c9d9daedf3885000a2ab4c6 100644
--- a/tools/gn/functions.cc
+++ b/tools/gn/functions.cc
@@ -468,7 +468,8 @@ Value RunImport(Scope* scope,
 
   const SourceDir& input_dir = scope->GetSourceDir();
   SourceFile import_file =
-      input_dir.ResolveRelativeFile(args[0].string_value());
+      input_dir.ResolveRelativeFile(args[0].string_value(),
+          scope->settings()->build_settings()->root_path());
   scope->settings()->import_manager().DoImport(import_file, function,
                                                scope, err);
   return Value();