Introduce sys_sigprocmask and sys_sigaction.

sandbox/linux/BUILD.gn

Index: sandbox/linux/BUILD.gn
diff --git a/sandbox/linux/BUILD.gn b/sandbox/linux/BUILD.gn
index 17941994f811b5fd084bfa896a1e4567fabc02b6..0db58434ddd70d3086cbb81587febbe4e33111a8 100644
--- a/sandbox/linux/BUILD.gn
+++ b/sandbox/linux/BUILD.gn
@@ -288,15 +288,16 @@ component("sandbox_services") {
 
 source_set("sandbox_services_headers") {
   sources = [
-    "system_headers/android_arm64_ucontext.h",
-    "system_headers/android_arm_ucontext.h",
     "system_headers/android_futex.h",
-    "system_headers/android_i386_ucontext.h",
-    "system_headers/android_ucontext.h",
     "system_headers/arm64_linux_syscalls.h",
+    "system_headers/arm64_linux_ucontext.h",
     "system_headers/arm_linux_syscalls.h",
+    "system_headers/arm_linux_ucontext.h",
+    "system_headers/i386_linux_ucontext.h",
     "system_headers/linux_seccomp.h",
+    "system_headers/linux_signal.h",
     "system_headers/linux_syscalls.h",
+    "system_headers/linux_ucontext.h",
     "system_headers/x86_32_linux_syscalls.h",
     "system_headers/x86_64_linux_syscalls.h",
   ]