Linux: add support for SELinux.

This patch adds support for a selinux GYP variable which, when set to
one, does the following:
  * Removes the seccomp sandbox from the compile
  * Removes support for SUID sandboxing from the zygote
  * Performs a dynamic transition, in the zygote, to
    chromium_renderer_t.

This code requires that the system policy have a sensible set of
access vectors for the chromium_renderer_t type. Such a policy will be
found in sandbox/selinux in the future.

http://codereview.chromium.org/203071


git-svn-id: svn://svn.chromium.org/chrome/trunk/src@26257 0039d316-1c4b-4281-b951-d872f2087c98