message: Add named cache for xcode installation.
  This will prevent the builders from having to install XCode on every run.
diff: @@ -70,6 +70,14 @@ builder_mixins {
  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 @@ buckets {
  # 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 @@ buckets: {
  # 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"
  }
  