The GPU mac builders have been consistently failing with the following error on gclient sync:

Deleting old toolchain.
Unpacking new toolchain.
Accepting license.
dyld: Library not loaded: /usr/lib/libcompression.dylib
  Referenced from: /b/c/b/GPU_Mac_Builder__dbg_/src/build/mac_files/Xcode.app/Contents/SharedFrameworks/DTXConnectionServices.framework/Versions/A/DTXConnectionServices
  Reason: image not found
Failed to download toolchain toolchain-8E2002-3.tgz.
Exception Command '['sudo', '/usr/bin/xcodebuild', '-license', 'accept']' returned non-zero exit status 133
Exiting.
Error: Command '/usr/bin/python src/build/mac_toolchain.py' returned non-zero exit status 1 in /b/c/b/GPU_Mac_Builder__dbg_
Hook '/usr/bin/python src/build/mac_toolchain.py' took 67.98 secs
step returned non-zero exit code: 2

The failure started with https://build.chromium.org/p/chromium.gpu/builders/GPU%20Mac%20Builder%20%28dbg%29/builds/100674.

I'm going to speculatively revert https://codereview.chromium.org/2941293003 since that looks like the most likely culprit in that range.