Use gpu-fyi-specific service account for these bots.

This is temporary, in order to be able to continue running the GLES1
and GLES2 conformance tests on these bots. The tests need to be
migrated to other machines.

Bug: 819751
Change-Id: Idfb9c70fd81cb7891bbc597e541a9bb64f686511
Reviewed-on: https://chromium-review.googlesource.com/953825Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541630}

infra/config/global/cr-buildbucket.cfg

@@ -142,6 +142,7 @@ builder_mixins {
 
 builder_mixins {
   name: "linux-gpu-fyi-ci"
+  service_account: "chromium-ci-gpu-builder@chops-service-accounts.iam.gserviceaccount.com"
   dimensions: "cpu:x86-64"
   dimensions: "cores:8"
   mixins: "linux"
@@ -173,6 +174,7 @@ builder_mixins {
 
 builder_mixins {
   name: "mac-gpu-fyi-ci"
+  service_account: "chromium-ci-gpu-builder@chops-service-accounts.iam.gserviceaccount.com"
   dimensions: "cpu:x86-64"
   mixins: "mac"
   recipe {