cc: Correct inverted directions in occlusion tracker background filter logic.

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 fe5b0a355e7d0e6ebb0e7535d4a4450b7b9a6ea7..09519af99f9e4fcc1ca605097908cf317b23fa97 100644
--- a/cc/trees/occlusion_tracker.cc
+++ b/cc/trees/occlusion_tracker.cc
@@ -211,9 +211,11 @@ static void ReduceOcclusionBelowSurface(
       &outset_top, &outset_right, &outset_bottom, &outset_left);
 
   // The filter can move pixels from outside of the clip, so allow affected_area
-  // to expand outside the clip.
-  affected_area_in_target.Inset(
-      -outset_left, -outset_top, -outset_right, -outset_bottom);
+  // to expand outside the clip. Notably the content we're concerned with here
+  // is not the affected area, but rather stuff slightly outside it. Thus the
+  // directions of the outsets are reversed from normal.
+  affected_area_in_target.Inset(-outset_right, -outset_bottom, -outset_left,
+                                -outset_top);
   SimpleEnclosedRegion affected_occlusion = *occlusion_from_inside_target;
   affected_occlusion.Intersect(affected_area_in_target);