Add namespace sandbox to about page.

This unindents the lines about PID/network namespaces, since those now apply to both the setuid or unprivileged namespace sandbox.

This is a resubmit of https://codereview.chromium.org/873283004/

TBR=jhawkins@chromium.org
BUG=312380

Review URL: https://codereview.chromium.org/916153003

Cr-Commit-Position: refs/heads/master@{#315945}
3 files changed