Roll mac_toolchain to d4dc0c2
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 <002a79d3522c3eaf360691f06af29425bafc4480@chromium.org>
Commit-Queue: Sergey Berezin <002a79d3522c3eaf360691f06af29425bafc4480@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools