Add "dumpsys SurfaceFlinger --dispsync"

Dumps the current DispSync state.

Bug 14651879

Change-Id: Ide4e6dbd58b117bc1a6b97b57d10cd92ec86dc84
diff --git a/services/surfaceflinger/DispSync.h b/services/surfaceflinger/DispSync.h
index 5826a78..a33ce5d 100644
--- a/services/surfaceflinger/DispSync.h
+++ b/services/surfaceflinger/DispSync.h
@@ -108,6 +108,9 @@
     // the refresh after next. etc.
     nsecs_t computeNextRefresh(int periodOffset) const;
 
+    // dump appends human-readable debug info to the result string.
+    void dump(String8& result) const;
+
 private:
 
     void updateModelLocked();