SurfaceFlinger: add debug information when vsync is not generated

Print debug information about VsyncReactor internal state when
we expect a vsync event but it is not generated.

Bug: 151892277
Test: adb shell dumpsys SurfaceFlinger --dispsync
Test: simulate a missed vsync and observed logcat
Change-Id: Ia98cb884321d0c25610c4d721f422d9cd05c8c5e
diff --git a/services/surfaceflinger/Scheduler/VSyncTracker.h b/services/surfaceflinger/Scheduler/VSyncTracker.h
index a25b8a9..05a6fc3 100644
--- a/services/surfaceflinger/Scheduler/VSyncTracker.h
+++ b/services/surfaceflinger/Scheduler/VSyncTracker.h
@@ -66,6 +66,8 @@
     /* Inform the tracker that the samples it has are not accurate for prediction. */
     virtual void resetModel() = 0;
 
+    virtual void dump(std::string& result) const = 0;
+
 protected:
     VSyncTracker(VSyncTracker const&) = delete;
     VSyncTracker& operator=(VSyncTracker const&) = delete;