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/InjectVSyncSource.h b/services/surfaceflinger/Scheduler/InjectVSyncSource.h
index fa46e6f..31da588 100644
--- a/services/surfaceflinger/Scheduler/InjectVSyncSource.h
+++ b/services/surfaceflinger/Scheduler/InjectVSyncSource.h
@@ -45,6 +45,7 @@
     const char* getName() const override { return "inject"; }
     void setVSyncEnabled(bool) override {}
     void setPhaseOffset(nsecs_t) override {}
+    void dump(std::string&) const override {}
 
 private:
     std::mutex mCallbackMutex;