Process GPU frequency info from ftrace if available

If a GPU driver outputs frequency information via ftrace on the
power/gpu_frequency event using the same format at used for CPU
frequency, process it as a counter named gpufreq.

Change-Id: I93644c0cdd8835668f2f3807b53c7bc429ec9d77
diff --git a/src/trace_processor/trace_storage.h b/src/trace_processor/trace_storage.h
index f757e25..6f31368 100644
--- a/src/trace_processor/trace_storage.h
+++ b/src/trace_processor/trace_storage.h
@@ -78,6 +78,7 @@
   kRefIrq = 3,
   kRefSoftIrq = 4,
   kRefUpid = 5,
+  kRefGpuId = 6,
   kRefMax
 };