[cr-buildbucket] Set chromium/Mac builder OS to MacOS 10.13, chromium/Linux to regular VM

Bug: 790269
TBR: tandrii
Change-Id: I45e883f69dd418be814a6ed96cb41e2e40b256d1
Reviewed-on: https://chromium-review.googlesource.com/1079895Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Ryan Tseng <hinoka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#563023}

infra/config/global/cr-buildbucket.cfg

@@ -1818,7 +1818,6 @@ buckets {
     builders {
       name: "Linux x64"
       dimensions: "os:Ubuntu-14.04"
-      dimensions: "cores:32"
       mixins: "chromium-ci"
     }
     builders {
@@ -1970,7 +1969,7 @@ buckets {
     }
     builders {
       name: "Mac"
-      dimensions: "os:Mac-10.12.6"
+      dimensions: "os:Mac-10.13"
       mixins: "chromium-ci"
     }
     builders {