Add pool dimension for linux-perf-fyi tests

Bug: 913625
Change-Id: I6d9f28e374b1751afcdd2ffb40d03d3c8ee87434
Reviewed-on: https://chromium-review.googlesource.com/c/1370495Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Ned Nguyen <nednguyen@google.com>
Cr-Commit-Position: refs/heads/master@{#615378}

testing/buildbot/chromium.perf.fyi.json

@@ -288,7 +288,8 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "id": "build186-b7"
+              "id": "build186-b7",
+              "pool": "chrome.tests.perf-fyi"
             }
           ],
           "expiration": 36000,