infra: Use macOS 10.15 and newer OSX SDK.

Bug: dawn:339
Change-Id: If25b3ace5ee6fe8d58054f9421a3417870dda750
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/16200
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
diff --git a/infra/config/global/cr-buildbucket.cfg b/infra/config/global/cr-buildbucket.cfg
index c5d47f7..1d45325 100644
--- a/infra/config/global/cr-buildbucket.cfg
+++ b/infra/config/global/cr-buildbucket.cfg
@@ -72,7 +72,7 @@
 }
 builder_mixins {
   name: "mac"
-  dimensions: "os:Mac-10.13"
+  dimensions: "os:Mac-10.15"
   caches: {   # cache for depot_tools.osx_sdk recipe module
     name: "osx_sdk"
     path: "osx_sdk"
@@ -80,7 +80,7 @@
   recipe {
     properties_j: <<EOF
       $depot_tools/osx_sdk:{
-        "sdk_version": "10b61"
+        "sdk_version": "11b52"
       }
     EOF
   }