layout: Improve our handling of inline absolute containing blocks.

Several issues are addressed in this commit:

* Inline flows now bubble up their absolute descendants instead of
  making the inline flow the containing block for them. (In the future,
  we will need to make the inline flow *sometimes* be the containing
  block for them, but for now it improves sites to unconditionally
  bubble up.)

* Fragments now look at their inline fragment context to determine
  whether they are positioned.

* Inline flows now push the stacking-relative position of the absolute
  containing block down to their inline-block fragments.

* Inline absolute hypothetical fragments can be containing blocks.

* Fixes the logic in
  `containing_block_range_for_flow_surrounding_fragment_at_index`. The
  condition to determine whether fragments are positioned was inverted!

* `Descendants`/`AbsDescendants` has been refactored in order to become
  more friendly to inline absolute containing blocks in the future.

Improves the inline position of the green drop-down arrow in the Google
SERPs. (The block position is still wrong.)

components/layout/fragment.rs

@@ -2234,6 +2234,22 @@ impl Fragment {
     pub fn margin_box_inline_size(&self) -> Au {
         self.border_box.size.inline + self.margin.inline_start_end()
     }
+
+    /// Returns true if this node *or any of the nodes within its inline fragment context* have
+    /// non-`static` `position`.
+    pub fn is_positioned(&self) -> bool {
+        if self.style.get_box().position != position::T::static_ {
+            return true
+        }
+        if let Some(ref inline_context) = self.inline_context {
+            for node in inline_context.nodes.iter() {
+                if node.style.get_box().position != position::T::static_ {
+                    return true
+                }
+            }
+        }
+        false
+    }
 }
 
 impl fmt::Debug for Fragment {