Specify gpu dimension for linux-perf-fyi

Bug:913625
Change-Id: Id6751f4af3aedac41fd3968a58a797b785cfbd84
TBR=jbudorick@chromium.org, bradhall@chromium.org

NOTRY=true # linux_chromium_rel_ng flake

Change-Id: Id6751f4af3aedac41fd3968a58a797b785cfbd84
Reviewed-on: https://chromium-review.googlesource.com/c/1371646
Commit-Queue: Ned Nguyen <nednguyen@google.com>
Reviewed-by: Ned Nguyen <nednguyen@google.com>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615540}

testing/buildbot/chromium.perf.fyi.json

@@ -288,6 +288,7 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
+              "gpu": "10de",
               "id": "build186-b7",
               "pool": "chrome.tests.perf-fyi"
             }