Better implement getComputedStyle() for positioned insets

The specification dictates quite quite idiosyncratic return values when
querying insets of positioned elements via getComputedStyle(). These
depend on whether or not the elements size was overconstrained. This
change adds a better implementation of that in preparation for returning
proper values for position: sticky.

components/layout_2020/fragment_tree/mod.rs

@@ -3,5 +3,7 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 mod base;
+mod containing_block;
 
 pub(crate) use base::*;
+pub(crate) use containing_block::*;