layout: Do not include position:fixed children when calculating scrollable overflow for root element (#38618)

Reimplementation of: #35931

For a `FragmentTree` we define a scrollable overflow calculation that
includes the overflow all of it's children `Fragments`. In practice we
are using this calculation for scrolling area of the viewport and
defining the root scroll frames. However, since uncontained fixed
positioned element is located outside of the document and should not be
scrolled, and therefore it would make no sense to include them in the
calculation of its scrollable overflow as well.

Testing: New and existing WPT tests
Fixes: #38617
Fixes: #38182

---------

Signed-off-by: Jo Steven Novaryo <jo.steven.novaryo@huawei.com>

tests/wpt/meta/MANIFEST.json

@@ -607585,6 +607585,13 @@
        }
       ]
      ],
+     "scrollable-overflow-fixed-positioned-initial-containing-block.html": [
+      "10f115a012a85230a4641a1d793c0a6df0db2e78",
+      [
+       null,
+       {}
+      ]
+     ],
      "scrollable-overflow-float.html": [
       "f75c0a66cfffdfe9872c6e472966ee3cf639eae6",
       [