Allow traced_probes to read /proc/ for other processes

Bug: 23310674
Change-Id: I8b40e0868c8905c8aa990064fe90e5fbc31bdc71
diff --git a/perfetto.rc b/perfetto.rc
index 8c15fd8..ece8862 100644
--- a/perfetto.rc
+++ b/perfetto.rc
@@ -25,7 +25,7 @@
     class late_start
     disabled
     user nobody
-    group shell
+    group shell readproc
     writepid /dev/cpuset/system-background/tasks
     # Clean up procfs configuration even if traced_probes crashes
     # unexpectedly.