[cr-buildbucket.cfg] Use Mac-10.13 for all chromium.memory mac builders

Bug: 790336, 790329
TBR: tandrii
Change-Id: I59216a9c02e079979c7e5e43b53801630558829b
Reviewed-on: https://chromium-review.googlesource.com/1109302Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Commit-Queue: Ryan Tseng <hinoka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569090}

infra/config/global/cr-buildbucket.cfg

@@ -1551,7 +1551,7 @@ buckets {
     }
     builders {
       name: "Mac ASan 64 Builder"
-      dimensions: "os:Mac-10.12.2"
+      dimensions: "os:Mac-10.13"
       mixins: "memory-ci"
     }
     builders {
@@ -1711,7 +1711,7 @@ buckets {
     }
     builders {
       name: "Mac ASan 64 Tests (1)"
-      dimensions: "os:Mac-10.9.5"
+      dimensions: "os:Mac-10.13"
       mixins: "memory-ci"
     }
     builders {