Make webrtc_perf_tests run serially on swarming.

BUG=chromium:755660
NOTRY=True

Review-Url: https://codereview.webrtc.org/3012973002
Cr-Commit-Position: refs/heads/master@{#19783}
diff --git a/tools_webrtc/mb/gn_isolate_map.pyl b/tools_webrtc/mb/gn_isolate_map.pyl
index e9eb538..e75e727 100644
--- a/tools_webrtc/mb/gn_isolate_map.pyl
+++ b/tools_webrtc/mb/gn_isolate_map.pyl
@@ -120,6 +120,6 @@
   },
   "webrtc_perf_tests": {
     "label": "//webrtc:webrtc_perf_tests",
-    "type": "console_test_launcher",
+    "type": "non_parallel_console_test_launcher",
   },
 }