surfaceflinger: add getDisplayStats() method

This is used by media service to schedule video frames at the
proper time, based on precise vsync timings.

Bug: 14659809
Change-Id: I1a90603f3dc09dca9aa4f90a3aa845fab56e0a5e
diff --git a/services/surfaceflinger/DispSync.h b/services/surfaceflinger/DispSync.h
index 7a26df3..96efc34 100644
--- a/services/surfaceflinger/DispSync.h
+++ b/services/surfaceflinger/DispSync.h
@@ -101,6 +101,9 @@
     // turned on.  It should NOT be used after that.
     void setPeriod(nsecs_t period);
 
+    // The getPeriod method returns the current vsync period.
+    nsecs_t getPeriod();
+
     // setRefreshSkipCount specifies an additional number of refresh
     // cycles to skip.  For example, on a 60Hz display, a skip count of 1
     // will result in events happening at 30Hz.  Default is zero.  The idea