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/VSyncDispatch.h b/services/surfaceflinger/Scheduler/VSyncDispatch.h
index a6fb3a4..2a2d7c5 100644
--- a/services/surfaceflinger/Scheduler/VSyncDispatch.h
+++ b/services/surfaceflinger/Scheduler/VSyncDispatch.h
@@ -108,6 +108,8 @@
      */
     virtual CancelResult cancel(CallbackToken token) = 0;
 
+    virtual void dump(std::string& result) const = 0;
+
 protected:
     VSyncDispatch() = default;
     VSyncDispatch(VSyncDispatch const&) = delete;