Add anchorElement element reference IDL

The anchor content attribute is not currently reflected in IDL. This CL
adds IDL reflection, as an element reference:

```
popover.anchorElement = myElement;
popover.setAttribute('anchor','idref');
const el = popover.anchorElement;
```

This is gated behind the CSSAnchorPositioning flag, since it is most
closely associated with that feature.

Bug: 1307772,1309178
Change-Id: I7c4d54d94ad5eeb38d1945733bbbaa3890f2b5d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4209776
Auto-Submit: Mason Freed <masonf@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1101192}

html/semantics/popovers/popover-anchor-idl-property.tentative.html

@@ -7,10 +7,41 @@
 
 <button id=b1>This is an anchor button</button>
 <div popover id=p1 anchor=b1>This is a popover</div>
-<button id=b2 popover=p1>This button invokes the popover but isn't an anchor</button>
+<button id=b2 popovertoggletarget=p1>This button invokes the popover but isn't an anchor</button>
 
 <script>
   test(function() {
-    assert_equals(p1.anchor,b1);
-  }, "popover anchor IDL property returns the anchor element");
+    assert_equals(p1.anchorElement,b1);
+  }, "popover anchorElement IDL property returns the anchor element");
+
+  test(function() {
+    assert_equals(p1.anchorElement,b1);
+    p1.anchorElement = b2;
+    assert_equals(p1.anchorElement,b2);
+    assert_equals(p1.getAttribute('anchor'),'','Idref is empty after setting element');
+    p1.anchorElement = b1; // Reset
+  }, "popover anchorElement is settable");
+</script>
+
+<button id=b1>button</button>
+<div id=p2>Anchored div</div>
+<style>
+  * {margin:0;padding:0;}
+  #b1 {width: 200px;}
+  #p2 {
+    position: absolute;
+    left: anchor(right);
+  }
+</style>
+
+<script>
+  test(function() {
+    assert_equals(p2.anchorElement,null);
+    const button = document.getElementById('b1');
+    assert_true(!!button);
+    p2.anchorElement = button;
+    assert_equals(p2.getAttribute('anchor'),'','Idref should be empty after setting element');
+    assert_equals(p2.anchorElement,button,'Element reference should be button');
+    assert_equals(p2.offsetLeft, 200, 'The anchor relationship should be functional');
+  }, "anchorElement affects anchor positioning");
 </script>