Run telemetry_unitests in serial

This is suspected to cause flaky failure of telemetry_unitests

BUG=661434

Review-Url: https://codereview.chromium.org/2564413002
Cr-Commit-Position: refs/heads/master@{#438143}

testing/buildbot/gn_isolate_map.pyl

@@ -841,6 +841,9 @@
       "--xvfb",
       "../../tools/perf/run_telemetry_tests",
       "-v",
+      # TODO(nedn, eyaich): Remove this flag once crbug.com/549140 is fixed &
+      # Telemetry no longer downloads files in parallel. (crbug.com/661434#c24)
+      "--jobs=1",
       "--chrome-root",
       "../../",
     ],