LibWeb: Rename Layout::Box::size() to content_size()

This property represents the CSS content size, so let's reduce ambiguity
by using the spec terminology.

We also bring a bunch of related functions along for the ride.

Userland/Libraries/LibWeb/Layout/InlineLevelIterator.cpp

@@ -102,7 +102,7 @@ Optional<InlineLevelIterator::Item> InlineLevelIterator::next(float available_wi
         .node = &box,
         .offset_in_node = 0,
         .length_in_node = 0,
-        .width = box.width(),
+        .width = box.content_width(),
     };
 }