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chrome fails to start with user namespace sandboxing:

[866:866:0120/134742:FATAL:zygote_host_impl_linux.cc(182)] Check failed: process.IsValid().
Failed to launch zygote process

With strace..

clone(child_stack=0x7ffd28fd98, flags=CLONE_NEWUSER|CLONE_NEWPID|CLONE_NEWNET|SIGCHLD) = -1 EINVAL (Invalid argument)

Which compared to kernel sources[1], shows that stack needs to be 16
bytes aligned. because stack grows downward, this patch assumes
PTHREAD_STACK_MIN is dividable by 16 too.

[1] http://lxr.free-electrons.com/source/arch/arm64/kernel/process.c#L267

BUG=581018
R=keescook@chromium.org,jln@chromium.org,rsesek@chromium.org,thakis@chromium.org
TEST=base_unittests ProcessUtilTest.* and sandbox_linux_unittests NamespaceSandboxTest.*

Review URL: https://codereview.chromium.org/1617763002

Cr-Commit-Position: refs/heads/master@{#371809}