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

cc/trees/occlusion_tracker.cc

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/trees/occlusion_tracker.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 
@@ skipping 193 matching lines @@
     affected_area_in_target.Intersect(contributing_surface->clip_rect());
   }
   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.
-  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);
 
   occlusion_from_inside_target->Subtract(affected_area_in_target);
   for (size_t i = 0; i < affected_occlusion.GetRegionComplexity(); ++i) {
     gfx::Rect occlusion_rect = affected_occlusion.GetRect(i);
 
     // Shrink the rect by expanding the non-opaque pixels outside the rect.
 
     // The left outset of the filters moves pixels on the right side of
@@ skipping 155 matching lines @@
   DCHECK(layer_tree->RootRenderSurface() == stack_.back().target);
   const SimpleEnclosedRegion& occluded =
       stack_.back().occlusion_from_inside_target;
   Region visible_region(screen_space_clip_rect_);
   for (size_t i = 0; i < occluded.GetRegionComplexity(); ++i)
     visible_region.Subtract(occluded.GetRect(i));
   return visible_region;
 }
 
 }  // namespace cc