Switch to flat tree traversal for <popup> ancestral element search

Previously, a light-DOM parent traversal was used, however, this was
already questionable, and it breaks the <selectmenu> use case. So I
think the right thing to do here is a flat tree traversal.

Bug: 1168738
Change-Id: I1008fed2a41d729c7dba79b878ed8f7845d95eb6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2893593
Auto-Submit: Mason Freed <masonf@chromium.org>
Commit-Queue: Ionel Popescu <iopopesc@microsoft.com>
Reviewed-by: Ionel Popescu <iopopesc@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#882861}

html/semantics/interactive-elements/the-popup-element/popup-shadow-dom.tentative.html

@@ -100,7 +100,7 @@
     <template shadowroot=open>
       <button id=t3b1 onclick='showPopup("test3",0)'>Test 3 Popup 1</button>
       <popup anchor=t3b1>
-        <p>This popup will be hidden when popup2 shows.</p>
+        <p>This popup will stay open when popup2 shows.</p>
         <slot></slot>
       </popup>
     </template>
@@ -115,15 +115,17 @@
     popup1.show();
     assert_true(popup1.open);
     assert_true(popupVisible(popup1));
-    // Showing popup2 will close popup1, since it is not a DOM
+    // Showing popup2 should not close popup1, since it is a flat
     // tree ancestor of popup2's anchor button.
     popup2.show();
     assert_true(popup2.open);
     assert_true(popupVisible(popup2));
+    assert_true(popup1.open);
+    assert_true(popupVisible(popup1));
+    popup1.hide();
+    assert_false(popup2.open);
     assert_false(popup1.open);
-    assert_false(popupVisible(popup1));
-    popup2.hide();
-  }, "anchor references use the DOM tree not the flat tree");
+  }, "anchor references use the flat tree not the DOM tree");
 </script>