Add assertion to check that CPU profiling actually works.

src/platform-linux.cc

Index: src/platform-linux.cc
diff --git a/src/platform-linux.cc b/src/platform-linux.cc
index 645b83377bb62e79dc4eb0ef0a723d94d2889137..e89d2cc9c8a861360fd3d704ae0baf650700d9f5 100644
--- a/src/platform-linux.cc
+++ b/src/platform-linux.cc
@@ -1208,7 +1208,9 @@ class SignalSender : public Thread {
 #if defined(ANDROID)
     syscall(__NR_tgkill, vm_tgid_, tid, SIGPROF);
 #else
-    syscall(SYS_tgkill, vm_tgid_, tid, SIGPROF);
+    int result = syscall(SYS_tgkill, vm_tgid_, tid, SIGPROF);
+    USE(result);
+    ASSERT(result == 0);
 #endif
   }