cc: early return if unoccluded_region_in_target_surface is empty

cc/trees/occlusion_tracker.cc

Index: cc/trees/occlusion_tracker.cc
diff --git a/cc/trees/occlusion_tracker.cc b/cc/trees/occlusion_tracker.cc
index f4a414f6de29c35815f8abf9592974f0c56a6e1a..fb3b5acfe94c90fc145aa1f44279270e1f7a453f 100644
--- a/cc/trees/occlusion_tracker.cc
+++ b/cc/trees/occlusion_tracker.cc
@@ -567,6 +567,9 @@ gfx::Rect OcclusionTracker<LayerType>::UnoccludedContentRect(
   unoccluded_region_in_target_surface.Subtract(
       stack_.back().occlusion_from_outside_target);
 
+  if (unoccluded_region_in_target_surface.IsEmpty())
+    return gfx::Rect();
+
   gfx::Rect unoccluded_rect_in_target_surface =
       unoccluded_region_in_target_surface.bounds();
   gfx::Rect unoccluded_rect = MathUtil::ProjectEnclosingClippedRect(
@@ -609,6 +612,9 @@ gfx::Rect OcclusionTracker<LayerType>::UnoccludedContributingSurfaceContentRect(
   unoccluded_region_in_target_surface.Subtract(
       second_last.occlusion_from_outside_target);
 
+  if (unoccluded_region_in_target_surface.IsEmpty())
+    return gfx::Rect();
+
   gfx::Rect unoccluded_rect_in_target_surface =
       unoccluded_region_in_target_surface.bounds();
   gfx::Rect unoccluded_rect = MathUtil::ProjectEnclosingClippedRect(