webservd: add pipe2 to seccomp policy

minijail crashes on this call have been observed in the CQ.

BUG=chromium:655780
TEST=verified webservd and local api still functional on whirlwind

Change-Id: I2c4ab5eabffa9208f5ee44e39e582366d1f0a183
Reviewed-on: https://chromium-review.googlesource.com/398484
Commit-Ready: Christopher Book <cbook@chromium.org>
Tested-by: Christopher Book <cbook@chromium.org>
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Reviewed-by: Garret Kelly <gdk@chromium.org>

webservd/usr/share/filters/webservd-seccomp.policy

diff --git a/webservd/usr/share/filters/webservd-seccomp.policy b/webservd/usr/share/filters/webservd-seccomp.policy
index 6cd8865..9c4b55e 100644
--- a/webservd/usr/share/filters/webservd-seccomp.policy
+++ b/webservd/usr/share/filters/webservd-seccomp.policy
@@ -51,6 +51,7 @@
 open: 1
 openat: 1
 pipe: 1
+pipe2: 1
 poll: 1
 prctl: 1
 read: 1