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: http://src.chromium.org/svn/trunk/src/build@241949 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -1915,6 +1915,17 @@
           }],
         ],
       }],
+      # The seccomp-bpf sandbox is only supported on three architectures
+      # currently.
+      # Do not disable seccomp_bpf anywhere without talking to
+      # security@chromium.org!
+      ['((OS=="linux" or OS=="android") and '
+           '(target_arch=="ia32" or target_arch=="x64" or '
+             'target_arch=="arm"))', {
+         'use_seccomp_bpf%': 1,
+      }, {
+         'use_seccomp_bpf%': 0,
+      }],
     ],
 
     # The path to the ANGLE library.