Fix FrameMetricsObserver crash

Bug: 30587465

Someday maybe the technology will exist to
allow sharing a simple constant between
Java and C++, but today is not that day.

Change-Id: I17694746cb8712058133cd5ea10c47b9909f740b
diff --git a/libs/hwui/FrameInfo.h b/libs/hwui/FrameInfo.h
index 1fe5459..45b57de 100644
--- a/libs/hwui/FrameInfo.h
+++ b/libs/hwui/FrameInfo.h
@@ -52,6 +52,7 @@
     QueueBufferDuration,
 
     // Must be the last value!
+    // Also must be kept in sync with FrameMetrics.java#FRAME_STATS_COUNT
     NumIndexes
 };