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 523b01f830f4a52408dbfb401a4f82ccbdaa4197..4cc21a8bec5422d4fc5022e31bfa7ade0f19a1fa 100644
--- a/cc/trees/occlusion_tracker.cc
+++ b/cc/trees/occlusion_tracker.cc
@@ -221,16 +221,16 @@ static void ReduceOcclusionBelowSurface(
   if (affected_area_in_target.IsEmpty())
     return;
 
-  int outset_top, outset_right, outset_bottom, outset_left;
-  contributing_surface->BackgroundFilters().GetOutsets(
-      &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. Notably the content we're concerned with here
   // is not the affected area, but rather stuff slightly outside it. Thus the

[danakj, 2017/05/19 19:28:54]

I think this comment doesn't explain what we have learnt very well. In my words
I'd say that the filter's bounds for a background filter are relative to the
target surface here rather than to the filtered surface, which means we have to
invert the outsets to apply them to the filtered surface's bounds. wdyt?

   // directions of the outsets are reversed from normal.
-  affected_area_in_target.Inset(-outset_right, -outset_bottom, -outset_left,
-                                -outset_top);
+  int outset_top, outset_right, outset_bottom, outset_left;
+  contributing_surface->BackgroundFilters().GetOutsets(
+      &outset_bottom, &outset_left, &outset_top, &outset_right);
+
+  affected_area_in_target.Inset(-outset_left, -outset_top, -outset_right,
+                                -outset_bottom);
   SimpleEnclosedRegion affected_occlusion = *occlusion_from_inside_target;
   affected_occlusion.Intersect(affected_area_in_target);