mac_chromium_compile_rel_ng is being upgraded to Mac 10.13.3.

R=hinoka@chromium.org

Bug: 816721, 817494
Change-Id: Ic39f00a60a246ab39e0547108a8ae3f057640220
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/941962Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539943}

infra/config/global/cr-buildbucket.cfg

@@ -524,7 +524,7 @@ buckets {
     builders { mixins: "mac-try" name: "mac_chromium_archive_rel_ng" }
     builders { mixins: "mac-try" name: "mac_chromium_asan_rel_ng" }
     builders { mixins: "mac-try" name: "mac_chromium_asan_variable" }
-    builders { mixins: "mac-try" name: "mac_chromium_compile_dbg_ng" dimensions:"os:Mac-10.12"}
+    builders { mixins: "mac-try" name: "mac_chromium_compile_dbg_ng" dimensions:"os:Mac-10.13.3"}
     builders { mixins: "mac-try" name: "mac_chromium_compile_rel_ng" }
     builders { mixins: "mac-try" name: "mac_chromium_dbg_ng" dimensions:"os:Mac-10.12"}
     builders { mixins: "mac-try" name: "mac_chromium_gn_upload" }