The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/40a8d84a54ac47d3299194c5377aebd71714f1f0

commit 40a8d84a54ac47d3299194c5377aebd71714f1f0
Author: Tom Anderson <thomasanderson@chromium.org>
Date: Fri Apr 06 20:34:58 2018

Enable thin archive builds on POSIX.

With the following gn args:
use_goma = true
is_debug = false
is_component_build = false

Reduces the size of the out directory from 2.4G to 1.8G and reduces
build time from 1m36s to 1m20s.

BUG= 801925 , 829956 

Change-Id: Ie426ae7b16385a166147bcb149694df7c93c2e14
Reviewed-on: https://chromium-review.googlesource.com/978648
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548918}
[modify] https://crrev.com/40a8d84a54ac47d3299194c5377aebd71714f1f0/build/config/compiler/BUILD.gn
[modify] https://crrev.com/40a8d84a54ac47d3299194c5377aebd71714f1f0/build/toolchain/gcc_ar_wrapper.py
[modify] https://crrev.com/40a8d84a54ac47d3299194c5377aebd71714f1f0/build/toolchain/gcc_toolchain.gni

we have updated to version r328903.

https://chromium-review.googlesource.com/c/chromiumos/overlays/chromiumos-overlay/+/1029211

why is a newer version of LLVM needed for this? 

actually, instead of closing this I should probably assign it to Thomas for further action. 

Thanks, marking this as fixed.  I'll enable thin archives on CrOs.