Seccomp BPF: initial structure for unittests

- On Linux, support a new sandbox_linux_unittests target
- Add a new seccomp_bpf_unittests target as a dependency of the global
  sandbox_linux_unittests target.
- Integrate globally as a chromium_builder_tests dependency


BUG=
TEST=


Review URL: https://chromiumcodereview.appspot.com/10635006

git-svn-id: http://src.chromium.org/svn/trunk/src/build@143549 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

all.gyp

@@ -227,6 +227,11 @@
             '../webkit/webkit.gyp:test_shell_common',
            ],
         }],
+        ['OS=="linux"', {
+          'dependencies': [
+            '../sandbox/sandbox.gyp:sandbox_linux_unittests',
+          ],
+        }],
         ['disable_nacl==0 and disable_nacl_untrusted==0', {
           'dependencies': [
             # TODO(bbudge): drop this when we have switched the NaCl proxy to IPC.