layout: Unify the block-size computation for blocks and floats.

The float code was old and did not support most of CSS 2.1. So unifying
the two paths both simplifies code and improves functionality.

Improves the Reddit sidebar.

components/layout/table_wrapper.rs

@@ -282,7 +282,7 @@ impl Flow for TableWrapperFlow {
         // Now compute the real value.
         let containing_block_inline_size = self.block_flow.base.position.size.inline;
         if self.is_float() {
-            self.block_flow.float.get_mut_ref().containing_inline_size =
+            self.block_flow.float.as_mut().unwrap().containing_inline_size =
                 containing_block_inline_size;
         }