ozone: fix HDPMLegacy - do the PF after overlays, also clear old overlays

ui/ozone/platform/dri/test/mock_dri_wrapper.cc

Index: ui/ozone/platform/dri/test/mock_dri_wrapper.cc
diff --git a/ui/ozone/platform/dri/test/mock_dri_wrapper.cc b/ui/ozone/platform/dri/test/mock_dri_wrapper.cc
index 3f65ffaaf2f41b818d7d1e6c3dcba723ac9fc804..e1265ad14f447e4f6a4980d444da05bc55a7aab8 100644
--- a/ui/ozone/platform/dri/test/mock_dri_wrapper.cc
+++ b/ui/ozone/platform/dri/test/mock_dri_wrapper.cc
@@ -69,6 +69,7 @@ MockDriWrapper::MockDriWrapper(int fd,
       remove_framebuffer_call_count_(0),
       page_flip_call_count_(0),
       overlay_flip_call_count_(0),
+      overlay_clear_call_count_(0),
       set_crtc_expectation_(true),
       add_framebuffer_expectation_(true),
       page_flip_expectation_(true),
@@ -153,6 +154,8 @@ bool MockDriWrapper::PageFlipOverlay(uint32_t crtc_id,
                                      const gfx::Rect& location,
                                      const gfx::Rect& source,
                                      int overlay_plane) {
+  if (!framebuffer)
+    overlay_clear_call_count_++;
   overlay_flip_call_count_++;
   return true;
 }