[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}

build/args/headless.gn

@@ -29,6 +29,10 @@ icu_use_data_file = false
 # to simplify deployment.
 v8_use_external_startup_data = false
 
+# Do not use bundled libc++ to ensure that everything works when Headless
+# Chromium is embedded.
+use_custom_libcxx = false
+
 enable_nacl = false
 enable_print_preview = false
 enable_remoting = false