layout: Simplify PositioningContext by having it hold a single Vec (#36795)

`PositioningContext` held two vectors, one inside an `Option`, to
differentiate between the version used for a containing block for all
descendants (including `position: absolute` and `position: fixed`) or
only for `position: absolute` descendants. This distinction was really
hard to reason about and required a lot of bookkeeping about what kind
of `PositioningContext` a layout box's parent expected. In addition, it
led to a lot of mistakes.

This change simplifies things so that `PositioningContext` only holds a
single vector. When it comes time to lay out hoisted absolutely
positioned
fragments, the code then:
 - lays out all of them (in the case of a `PositioningContext` for all
   descendants), or
 - only lays out the `position: absolute` descendants and preserves the
   `position: fixed` descendants (in the case the `PositioningContext`
   is only for `position: absolute`.), or
- lays out none of them if the `PositioningContext` was created for
  box that did not establish a containing block for absolutes.

It's possible that this way of dealing with hoisted absolutes is a bit
less efficient, but, the number of these descendants is typically quite
small, so it should not be significant. In addition, this decreases the
size in memory of all `PositioningContexts` which are created in more
situations as time goes on.

Testing: There is a new WPT test with this change.
Fixes: #36696.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

tests/wpt/meta/MANIFEST.json

@@ -2794,6 +2794,13 @@
     },
     "css-grid": {
      "abspos": {
+      "abspos-in-flexbox-in-grid-crash.html": [
+       "a2f71fb78e7f6fd82ea539e3af6c76d36f21e134",
+       [
+        null,
+        {}
+       ]
+      ],
       "positioned-grid-items-crash.html": [
        "228a6264df77fe82587a4116f231ff09537c8372",
        [