This provides a mechanism for a renderer to change the priority of
its threads and as a result also the cpuset they are assigned to.

A ChildProcessHost IPC message from the renderer is used to request
a thread priority change and have the browser process identify the
renderer thread that is requesting a priority change by checking all
its threads to find a thread with NSpid field in
/proc/[pid]/task/[thread_id]/status that matches the namespace tid
from the renderer.

This is currently limited to Linux and ChromeOS but follow up work
will investigate the possibility and benefits of doing the same on
other platforms.

Note: Thread priorities in the renderer are already adjusted in a
similar way on Android as the sandbox is not strict enough to
prevent this on Android today.

BUG=chrome-os-partner:56550
TEST=

Review-Url: https://codereview.chromium.org/2334533002
Cr-Commit-Position: refs/heads/master@{#419649}