Enable -Wshift-negative-value.

sandbox/linux/BUILD.gn

Index: sandbox/linux/BUILD.gn
diff --git a/sandbox/linux/BUILD.gn b/sandbox/linux/BUILD.gn
index 3e98defa5c612f298631edb395a8146c82436edf..1dab49ca44d328ffeec2e76542e2d9abf638beae 100644
--- a/sandbox/linux/BUILD.gn
+++ b/sandbox/linux/BUILD.gn
@@ -146,6 +146,11 @@ source_set("sandbox_linux_unittests_sources") {
       "seccomp-bpf/trap_unittest.cc",
     ]
     deps += [ ":bpf_dsl_golden" ]
+
+    # syscall_parameters_restrictions_unittests.cc uses MAKE_PROCESS_CPUCLOCK,
+    # which shifts signed ints around, which has undefined behavior.
+    # TODO(thakis): Fix that in LSS, then remove this flag here.
+    cflags = [ "-Wno-shift-negative-value" ]
   }
   if (compile_credentials) {
     sources += [