surfaceflinger: add frame duration logging

Change-Id: Ib414a45e7e191f23a2726cbbbeb606e9ce68a3b5
diff --git a/services/surfaceflinger/Layer.h b/services/surfaceflinger/Layer.h
index 0ceb15e..9093116 100644
--- a/services/surfaceflinger/Layer.h
+++ b/services/surfaceflinger/Layer.h
@@ -298,6 +298,7 @@
     void dump(String8& result, Colorizer& colorizer) const;
     void dumpStats(String8& result) const;
     void clearStats();
+    void logFrameStats();
 
 protected:
     // constant