Update offsetParent behavior in shadow trees

New behavior for offsetParent in shadow trees was discussed in [1] and
[2]. This lead to a chromium patch [3] which changed the behavior. After
[3] landed, the desired behavior in the discussions in [1] and [2]
seemed to have changed. Unfortunately, the author of [3] was no longer
working on chromium. This new behavior was then added to the spec and
landed in webkit [4] and firefox [5], and a WPT was added for it [6].

This patch implements the new behavior to follow suit with webkit and
firefox based on the WPT in [6]. Unfortunately, there are several tests
which are either internal to chromium or are only passing in chromium
which appear to oppose this new behavior and will have to be updated or
removed:
- external/wpt/css/css-contain/content-visibility/content-visibility-035.html
- external/wpt/css/css-contain/content-visibility/content-visibility-044.html
- fast/dom/shadow/offset-parent-does-not-leak-ua-shadow.html
- shadow-dom/offsetParent.html
For shadow-dom/offsetParent.html, I verified that firefox and safari
both currently fail the same tests which this patch does.

[1] WICG/webcomponents#497
[2] w3c/csswg-drafts#159
[3] https://chromium.googlesource.com/chromium/src/+/18d455ee833f6a30dcbe2755380861eb75cd9f6f
[4] https://trac.webkit.org/changeset/239313/webkit
[5] https://bugzilla.mozilla.org/show_bug.cgi?id=1514074
[6] https://wpt.fyi/results/shadow-dom/offsetParent-across-shadow-boundaries.html

Fixed: 920069
Change-Id: I168edc5ad0e4fcb92d0c4a440623f2424b14a988

css/css-contain/content-visibility/content-visibility-035.html

@@ -41,10 +41,10 @@
   }
 
   async function forceLayoutTest() {
-    t.step(() => assert_equals(slotted.offsetTop, 0));
+    t.step(() => assert_equals(slotted.offsetTop, 8));
     // Add a 20px div above the slotted div.
     container.innerHTML = "<div style='height: 20px;'></div><slot></slot>";
-    t.step(() => assert_equals(slotted.offsetTop, 20));
+    t.step(() => assert_equals(slotted.offsetTop, 28));
     t.done();
   }
 

css/css-contain/content-visibility/content-visibility-044.html

@@ -23,7 +23,7 @@
   t.step(async () => {
     container.classList.add("hidden");
     requestAnimationFrame(() => {
-      assert_equals(slotted.offsetTop, 0);
+      assert_equals(slotted.offsetTop, 8);
       container.innerHTML = "<div style='height: 20px;'></div><slot></slot>";
       t.done();
     });