Disable parallelization of telemetry_perf_unittest on all Mac bots

Bug: 753495
Change-Id: I0121d82ce25bc037c20914b74714aec076aa8fe3
Reviewed-on: https://chromium-review.googlesource.com/763873Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Ned Nguyen <nednguyen@google.com>
Cr-Commit-Position: refs/heads/master@{#516112}

testing/buildbot/chromium.mac.json

@@ -408,6 +408,9 @@
         }
       },
       {
+        "args": [
+          "--jobs=1"
+        ],
         "isolate_name": "telemetry_perf_unittests",
         "name": "telemetry_perf_unittests",
         "swarming": {
@@ -879,6 +882,9 @@
         }
       },
       {
+        "args": [
+          "--jobs=1"
+        ],
         "isolate_name": "telemetry_perf_unittests",
         "name": "telemetry_perf_unittests",
         "swarming": {