Make building seccomp-bpf a GYP condition

We add a use_seccomp_bpf variable to allow not building
seccomp-bpf.

We make sure that it can't be use by mistake with sanity
checks that the architecture is MIPS.

BUG=267179
TEST=Use "GYP_DEFINES='use_seccomp_bpf=0' and comment out the two #error.
about:sandbox should say that seccomp-bpf is disabled.

Review URL: https://codereview.chromium.org/117803002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241949 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241949
Revert of https://codereview.chromium.org/86443002/
Reason for revert: Breaks Android Performance Bots http://build.chromium.org/p/chromium.perf/builders/Android%20Nexus10%20Perf/builds/17

TBR=dtrainor@chromium.org,tedchoc@chromium.org
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/98413005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241950 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241950