bisect-kit: work around undefined variables in DEPS file

BUG=b:182219439
TEST=None

Change-Id: I090ff62679311e4f6bd811a435252eb29d0cf750
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/bisect-kit/+/2764117
Reviewed-by: Zheng-Jie Chang <zjchang@chromium.org>
Commit-Queue: Kuang-che Wu <kcwu@chromium.org>
Tested-by: Kuang-che Wu <kcwu@chromium.org>
diff --git a/bisect_kit/gclient_util.py b/bisect_kit/gclient_util.py
index 618450c..737d555 100644
--- a/bisect_kit/gclient_util.py
+++ b/bisect_kit/gclient_util.py
@@ -337,6 +337,13 @@
         'True': True,
     }
     vars_dict.update(self.variables)
+
+    # Work around b/182219439
+    # TODO(kcwu): mimic gclient's behavior, which convert undefined variables
+    # to strings
+    if 'use_rts' not in vars_dict:
+      vars_dict['use_rts'] = True
+
     # pylint: disable=eval-used
     return eval(self.condition, vars_dict)