[build] Disable -Wthread-safety under nacl

R=thakis@chromium.org

Bug: 982423
Change-Id: Ic051da1ad281198d3c3e594c9ded63e5802895f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1695681
Commit-Queue: Gabriel Charette <gab@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#676005}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 53c9abc0710023db8ce854f2d514b5ae67307ca6
1 file changed