Tree counting functions should not operate on the flat tree

Per resolution and PR[1]

This is basically a revert of [2] and [3].

[1] w3c/csswg-drafts#12144
[2] https://crrev.com/09ec78e0186d6d28678452d305af13ce4cf092dd
[3] https://crrev.com/335f039762e05639ba4f0efc00022b36a631e259

Bug: 413015181
Change-Id: I50b38a60745f85eae0346556863def756bd8f427
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6508508
Commit-Queue: Rune Lillesveen 
Reviewed-by: Steinar H Gunderson 
Cr-Commit-Position: refs/heads/main@{#1454908}

Author: lilles

css/css-values/tree-counting/calc-sibling-function-in-shadow-dom.html

@@ -33,9 +33,9 @@
          
        
       `;
-      assert_equals(getComputedStyle(target).zIndex, '3');
-      assert_equals(getComputedStyle(target).order, '3');
-      assert_equals(getComputedStyle(target).orphans, '3');
-      assert_equals(getComputedStyle(target).widows, '3');
-    }, 'Host children have sibling-index() and sibling-count() based on assignedNodes order');
+      assert_equals(getComputedStyle(target).zIndex, '4');
+      assert_equals(getComputedStyle(target).order, '4');
+      assert_equals(getComputedStyle(target).orphans, '4');
+      assert_equals(getComputedStyle(target).widows, '4');
+    }, 'Host children have sibling-index() and sibling-count() based on the DOM tree order');
 script>

css/css-values/tree-counting/sibling-function-invalidation.html

@@ -48,55 +48,3 @@
     assert_equals(t2.offsetHeight, 50);
   }, "5 siblings after removal");
 script>
-
-<style>
-  #t3 {
-    width: calc(10px * sibling-index());
-    height: 50px;
-    background: teal;
-  }
-style>
-<div>
-  <template shadowrootmode="open">
-    <slot>slot>
-  template>
-  <div id="rm3">div>
-  <div>div>
-  <div>div>
-  <div>div>
-  <div>div>
-  <div id="t3">div>
-div>
-<script>
-  test(() => assert_equals(t3.offsetWidth, 60), "Initially 6th slotted sibling");
-  test(() => {
-    rm3.slot = "null";
-    assert_equals(t3.offsetWidth, 50);
-  }, "5th sibling after slot change");
-script>
-
-<style>
-  #t4 {
-    width: 50px;
-    height: calc(10px * sibling-count());
-    background: teal;
-  }
-style>
-<div>
-  <template shadowrootmode="open">
-    <slot>slot>
-  template>
-  <div id="t4">div>
-  <div>div>
-  <div>div>
-  <div>div>
-  <div>div>
-  <div id="rm4">div>
-div>
-<script>
-  test(() => assert_equals(t4.offsetHeight, 60), "Initially 6 slotted siblings");
-  test(() => {
-    rm4.slot = "null";
-    assert_equals(t4.offsetHeight, 50);
-  }, "5 siblings after slot change");
-script>