Bug 1290761 - Allow running jprof with sandbox enabled. r=jhector

jprof is an in-tree profiling tool that runs on Linux.

This fixes the error:

Sandbox: seccomp sandbox violation: pid 29698, syscall 38, args 0 140731305513136 0 830 22509600 1.  Killing process.
Sandbox: crash reporter is disabled (or failed); trying stack trace:
Sandbox: frame #1: __GI_setitimer (/build/glibc-GKVZIf/glibc-2.23/time/../sysdeps/unix/syscall-template.S:84)
Sandbox: frame #2: startSignalCounter(unsigned long) (.../mozilla-central/mozilla/tools/jprof/stub/libmalloc.cpp:464)

which occurs during shutdown when running with jprof enabled via the
JPROF_FLAGS environment variable containing JP_DEFER without actually
sending the signal to start jprof.  It presumably occurs sooner if jprof
is actually used either via JP_START or by senging a SIGPROF/SIGALRM.

With the patch, these steps run to completion.

MozReview-Commit-ID: Fx4tzEyqIj2

UltraBlame original commit: eadaa06966fefa5ea284da51a8d58ef2423edf47

security/sandbox/linux/SandboxFilter.cpp

@@ -700,6 +700,11 @@ class ContentSandboxPolicy : public SandboxPolicyCommon {
 #endif
       return Allow();
 
+#ifdef MOZ_JPROF
+    case __NR_setitimer:
+      return Allow();
+#endif 
+
     default:
       return SandboxPolicyCommon::EvaluateSyscall(sysno);
     }