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/tests/unittests/EventThreadTest.cpp b/services/surfaceflinger/tests/unittests/EventThreadTest.cpp
index 65b3e35..ba5c0c2 100644
--- a/services/surfaceflinger/tests/unittests/EventThreadTest.cpp
+++ b/services/surfaceflinger/tests/unittests/EventThreadTest.cpp
@@ -48,6 +48,7 @@
     MOCK_METHOD1(setCallback, void(VSyncSource::Callback*));
     MOCK_METHOD1(setPhaseOffset, void(nsecs_t));
     MOCK_METHOD1(pauseVsyncCallback, void(bool));
+    MOCK_CONST_METHOD1(dump, void(std::string&));
 };
 
 } // namespace