Roll mac_toolchain to 496a91484
Roll mac_toolchain to 496a91484

Relevant change:
  496a91484 mac_toolchain: preserve u+w and mtime for Xcode

Bug: 797051
Change-Id: Iae5b34ff31e110ce827a1d94dcee86049c5d7bc4
TBR=iannucci@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/998624
Reviewed-by: Sergey Berezin <002a79d3522c3eaf360691f06af29425bafc4480@chromium.org>
Commit-Queue: Sergey Berezin <002a79d3522c3eaf360691f06af29425bafc4480@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools