Pin loading.desktop.network_service perf tests to a specific bot.

So that we could get more consistent data across runs.

BUG=717744

Change-Id: I78eba114b1d4b603448725fefcc20ce3a3e1db71
Reviewed-on: https://chromium-review.googlesource.com/919314Reviewed-by: Ned Nguyen <nednguyen@google.com>
Commit-Queue: Yuzhu Shen <yzshen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537184}

testing/buildbot/chromium.perf.fyi.json

@@ -418,6 +418,12 @@
         ],
         "swarming": {
           "can_use_on_swarming_builders": true,
+          "dimension_sets": [
+            {
+              "id": "build113-b4",
+              "pool": "Chrome-perf-fyi"
+            }
+          ],
           "expiration": 36000,
           "hard_timeout": 10800,
           "ignore_task_failure": false,