depot_tools: bump mac_toolchain version

Relevant change:
  38eded8ba mac_toolchain: add -service-account-json option

R=iannucci@chromium.org
BUG=475693

Change-Id: I89f8c53bf4bbdfa33b08647ed36f4baad0fa2349
Reviewed-on: https://chromium-review.googlesource.com/762059
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Sergey Berezin <sergeyberezin@chromium.org>

cipd_manifest.txt

@@ -14,4 +14,4 @@ infra/tools/luci/vpython/${platform}  git_revision:a3d636052088db3daa48413b3e209
 infra/tools/luci/led/${platform}  git_revision:e6c2ae3a0a56b33ae5426c04b1f3412fe853d214
 
 # Mac toolchain installer
-infra/tools/mac_toolchain/${os=mac}-${arch} git_revision:7eb9e7d8efe8bfec749e364470c5e7a7260c9ac2
+infra/tools/mac_toolchain/${os=mac}-${arch} git_revision:38eded8baee0633bffd47c1f3028006e7c84661e