Roll mac_toolchain to 478dc9dbb

Relevant changes:
  [mac_toolchain] Always call runFirstLaunch for Xcode 9+.

Bug: 841910
TBR=iannucci@chromium.org
Change-Id: I3f4b8fb793e66315492f8e53832e0ac9d00ceb0c
Reviewed-on: https://chromium-review.googlesource.com/1072007
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Sergey Berezin <sergeyberezin@chromium.org>

cipd_manifest.txt

@@ -14,7 +14,7 @@ infra/tools/luci/vpython/${platform}  git_revision:4d19637ec2c3d1efd8c6a1b052851
 infra/tools/luci/led/${platform}  git_revision:7c35171f0bf2061e5c231e5f4df1982dfa8066a6
 
 # Mac toolchain installer
-infra/tools/mac_toolchain/${os=mac}-${arch} git_revision:496a91484cdf28782a5580c120bca27881824b66
+infra/tools/mac_toolchain/${os=mac}-${arch} git_revision:478dc9dbbaa60dd9827f2b1106c5e70f1f6b429c
 
 # LUCI rpc command line tool
 infra/tools/prpc/${platform} git_revision:e1abc57be62d198b5c2f487bfb2fa2d2eb0e867c