[headless] Use system libc++ for headless buildbot.

R=eseckler@chromium.org
BUG=615047

Change-Id: I27cca41d2ea79b07f34d48c1bc1091cfd6caa9d9
Reviewed-on: https://chromium-review.googlesource.com/570242
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Commit-Queue: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486386}
1 file changed