Roll mac_toolchain to d4dc0c2

Relevant commits:
  d4dc0c29a mac_toolchain: add support for legacy Xcode 8.*

BUG=797051
TBR=vadimsh@chromium.org

Change-Id: I79a77a03b332d7f4bfbe8208928d67194dfc1854
Reviewed-on: https://chromium-review.googlesource.com/897685
Reviewed-by: Sergey Berezin <sergeyberezin@chromium.org>
Commit-Queue: Sergey Berezin <sergeyberezin@chromium.org>

cipd_manifest.txt

@@ -14,4 +14,4 @@ infra/tools/luci/vpython/${platform}  git_revision:27f5f70f88c13f5af0f05171a3b70
 infra/tools/luci/led/${platform}  git_revision:54b6b3102fbb15ca3e3bd762ff3df7b08057d653
 
 # Mac toolchain installer
-infra/tools/mac_toolchain/${os=mac}-${arch} git_revision:2b69be6203f56a970202d9b5984557c0453b4eb0
+infra/tools/mac_toolchain/${os=mac}-${arch} git_revision:d4dc0c29a004b59dcca1e69ae84eba0c932eb010