Update the os dimension for Mac10.(10|11) Tests builds.

Their build machines (*not* test machines) were upgraded to 10.13.

R=jbudorick, martiniss

Bug: 943050
Change-Id: Ia59873760150ca493a9700cb496f881db115a45d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1531065Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642279}

infra/config/cr-buildbucket.cfg

@@ -1395,12 +1395,14 @@ buckets {
     builders {
       name: "Mac10.10 Tests"
       mixins: "mac-ci"
-      dimensions:"os:Mac-10.10"
+      # The build runs on 10.13, but triggers tests on 10.10 bots.
+      dimensions:"os:Mac-10.13"
     }
     builders {
       name: "Mac10.11 Tests"
       mixins: "mac-ci"
-      dimensions:"os:Mac-10.11"
+      # The build runs on 10.13, but triggers tests on 10.11 bots.
+      dimensions:"os:Mac-10.13"
     }
     builders {
       name: "Mac10.12 Tests"