This uses the macOS<->XCode mapping from wikipedia to set the default XCode
version to the highest supported XCode version supported by the current
host OS.

R=justincohen@google.com, tandrii@chromium.org

Bug: 1026340
Recipe-Nontrivial-Roll: build
Recipe-Nontrivial-Roll: infra
Recipe-Nontrivial-Roll: build_limited_scripts_slave
Change-Id: I524d08f81a7778404c60a64b1100b4386a2ed79a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1938022Reviewed-by: Andrii Shyshkalov <tandrii@google.com>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>