Improve overdraw with multiple windows

cc/trees/occlusion_tracker_unittest.cc

Index: cc/trees/occlusion_tracker_unittest.cc
diff --git a/cc/trees/occlusion_tracker_unittest.cc b/cc/trees/occlusion_tracker_unittest.cc
index 8389954958e4689b8ff28fc37e6b81cb151a878e..a206aadacf80b3e4860b9530096934c65fbc82e7 100644
--- a/cc/trees/occlusion_tracker_unittest.cc
+++ b/cc/trees/occlusion_tracker_unittest.cc
@@ -1465,8 +1465,13 @@ class OcclusionTrackerTestPixelsNeededForDropShadowBackgroundFilter
         case LEFT:
           // The right half of the occlusion is close enough to cast a shadow
           // that would be visible in the background filter. The shadow reaches
-          // 3*5 + 10 = 25 pixels to the right.
-          expected_occlusion = gfx::Rect(0, 0, 25, 200);
+          // 3*5 + 10 = 25 pixels to the right, which is (0, 0, 25, 200). The
+          // rect created in filtered_surface is (50, 50, 100, 100), which
+          // becomes (25, 25, 20, 80) after applying gfx::Transform
+          // |scale_by_half|, adding FilterOperations |filters| and updating
+          // opacity pixel outside the rect. Union (0, 0, 25, 200) and
+          // (25, 25, 20, 80), it's (0, 25, 45, 80)
+          expected_occlusion = gfx::Rect(0, 25, 45, 80);
           break;
         case RIGHT:
           // The shadow spreads 3*5 - 10 = 5 pixels to the left, so the
@@ -1475,7 +1480,7 @@ class OcclusionTrackerTestPixelsNeededForDropShadowBackgroundFilter
           break;
         case TOP:
           // Similar to LEFT.
-          expected_occlusion = gfx::Rect(0, 0, 200, 25);
+          expected_occlusion = gfx::Rect(25, 0, 80, 45);
           break;
         case BOTTOM:
           // Similar to RIGHT.