Introduce sys_sigprocmask and sys_sigaction.

sandbox/linux/services/credentials.cc

Index: sandbox/linux/services/credentials.cc
diff --git a/sandbox/linux/services/credentials.cc b/sandbox/linux/services/credentials.cc
index 475ce3f1b033a999867bf1608eeee6540d1469d4..a1014bfe058370b794d9c6a88d2aba14a3a9750b 100644
--- a/sandbox/linux/services/credentials.cc
+++ b/sandbox/linux/services/credentials.cc
@@ -27,16 +27,12 @@
 #include "sandbox/linux/services/syscall_wrappers.h"
 #include "sandbox/linux/services/thread_helpers.h"
 #include "sandbox/linux/system_headers/capability.h"
+#include "sandbox/linux/system_headers/linux_signal.h"
 
 namespace sandbox {
 
 namespace {
 
-// Signal ABI for some toolchain is incompatible with Linux's. In particular,
-// PNaCl toolchain defines SIGCHLD = 20. So, here, directly define Linux's
-// value.
-const int kLinuxSIGCHLD = 17;
-
 bool IsRunningOnValgrind() { return RUNNING_ON_VALGRIND; }
 
 // Checks that the set of RES-uids and the set of RES-gids have
@@ -96,7 +92,7 @@ bool ChrootToSafeEmptyDir() {
 #endif
 
   pid = clone(ChrootToSelfFdinfo, stack,
-              CLONE_VM | CLONE_VFORK | CLONE_FS | kLinuxSIGCHLD,
+              CLONE_VM | CLONE_VFORK | CLONE_FS | LINUX_SIGCHLD,
               nullptr, nullptr, nullptr, nullptr);
   PCHECK(pid != -1);