Add a tag for tracing system server.

This change is needed for
https://android-review.googlesource.com/#/c/157562/

BUG: 21739901
Change-Id: Ic4616065749236fbde9263b8f1898c434913564a
diff --git a/cmds/atrace/atrace.cpp b/cmds/atrace/atrace.cpp
index 5a33417..e3dbbd5 100644
--- a/cmds/atrace/atrace.cpp
+++ b/cmds/atrace/atrace.cpp
@@ -92,6 +92,7 @@
     { "bionic",     "Bionic C Library", ATRACE_TAG_BIONIC, { } },
     { "power",      "Power Management", ATRACE_TAG_POWER, { } },
     { "pm",         "Package Manager",  ATRACE_TAG_PACKAGE_MANAGER, { } },
+    { "ss",         "System Server",    ATRACE_TAG_SYSTEM_SERVER, { } },
     { "sched",      "CPU Scheduling",   0, {
         { REQ,      "/sys/kernel/debug/tracing/events/sched/sched_switch/enable" },
         { REQ,      "/sys/kernel/debug/tracing/events/sched/sched_wakeup/enable" },