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 0d60d688adc47980b20123e58cc6d8d85656e20d..84d5ea4312b4fab42ec7c037961a80e24b4d87f7 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_utf8());
   scope->settings()->import_manager().DoImport(import_file, function,
                                                scope, err);
   return Value();