cc: Fix skewport math

cc/resources/picture_layer_tiling.cc

Index: cc/resources/picture_layer_tiling.cc
diff --git a/cc/resources/picture_layer_tiling.cc b/cc/resources/picture_layer_tiling.cc
index b52ba9cdf13fbe5b37f85047ebf71b138d48357e..8c5d88c7df25e2d9f603384f3fc16a44482fdc8b 100644
--- a/cc/resources/picture_layer_tiling.cc
+++ b/cc/resources/picture_layer_tiling.cc
@@ -564,11 +564,17 @@ gfx::Rect PictureLayerTiling::ComputeSkewport(
                  extrapolation_multiplier * (old_right - new_right),
                  extrapolation_multiplier * (old_bottom - new_bottom));
 
-  // Clip the skewport to |max_skewport|.
+  // Ensure that visible rect is contained in the skewport.
+  skewport.Union(visible_rect_in_content_space);
+
+  // Clip the skewport to |max_skewport|. This needs to happen after the
+  // union in case intersecting would have left the empty rect.
   skewport.Intersect(max_skewport);
 
-  // Finally, ensure that visible rect is contained in the skewport.
-  skewport.Union(visible_rect_in_content_space);
+  // The max_skewport should always contain the visible_rect_in_content_space
+  // so this should still be true.
+  DCHECK(skewport.Contains(visible_rect_in_content_space));
+
   return skewport;
 }