Add named cache for xcode installation.

This will prevent the builders from having to install XCode on every run.

R=cwallez@chromium.org, tandrii@google.com

Change-Id: I6480968dbf7d7f05bfd670ccbbf4790eb0c21d86
Reviewed-on: https://dawn-review.googlesource.com/c/5000
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@google.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

infra/config/global/cr-buildbucket.cfg

diff --git a/infra/config/global/cr-buildbucket.cfg b/infra/config/global/cr-buildbucket.cfg
index 1c78612..19864c7 100644
--- a/infra/config/global/cr-buildbucket.cfg
+++ b/infra/config/global/cr-buildbucket.cfg
@@ -70,6 +70,14 @@
     properties: "target_cpu:x64"
   }
 }
+builder_mixins {
+  name: "mac"
+  dimensions: "os:Mac-10.13"
+  caches: {   # cache for depot_tools.osx_sdk recipe module
+    name: "osx_sdk"
+    path: "osx_sdk"
+  }
+}
 
 buckets {
   name: "luci.dawn.ci"
@@ -122,12 +130,12 @@
     # Mac: unlike Linux we only need to test x64 (and only clang too)
     builders {
       name: "mac-dbg"
-      dimensions: "os:Mac-10.13"
+      mixins: "mac"
       mixins: "debug"
     }
     builders {
       name: "mac-rel"
-      dimensions: "os:Mac-10.13"
+      mixins: "mac"
       mixins: "release"
     }
 
@@ -248,13 +256,13 @@
     # Actually on the CQ
     builders {
       name: "mac-dbg"
-      dimensions: "os:Mac-10.13"
+      mixins: "mac"
       mixins: "debug"
     }
     # Actually on the CQ
     builders {
       name: "mac-rel"
-      dimensions: "os:Mac-10.13"
+      mixins: "mac"
       mixins: "release"
     }