Make ShadowRoot.elementsFromPoint always return document element

Currently in some cases, ShadowRoot.elementsFromPoint may not return
the document element even when Document.elementsFromPoint returns the
document element such as when the document element height is shorter than
the hit-tested element's position. This CL fixes it by removing a check
in TreeScope::ElementsFromHitTestResult.

See example live case detailed in this comment:
https://bugs.chromium.org/p/chromium/issues/detail?id=843215#c16

Bug: 843215
Change-Id: I5b774d1a091f3c3f72345c55bca188fad309ba58
Reviewed-on: https://chromium-review.googlesource.com/1131023
Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574104}

shadow-dom/DocumentOrShadowRoot-prototype-elementFromPoint.html

@@ -29,6 +29,11 @@
 
 const displayValues = ['inline', 'block', 'inline-block'];
 var container = document.getElementById('container');
+customElements.define('test-element', class extends HTMLElement {
+  constructor() {
+    super();
+  }
+});
 
 displayValues.forEach(function (displayValue) {
   test(function () {
@@ -228,6 +233,21 @@
   }, 'document.elementsFromPoint must return the shadow host and its ancestors and shadowRoot.elementsFromPoint must return the slot parent of the fallback text and its non-shadow ancestors when the hit-tested text node is a fallback content and the host has display: ' + displayValue);
 });
 
+test(function () {
+  container.innerHTML = '';
+  let host = document.createElement('test-element');
+  host.style.display = 'block';
+  let shadow = host.attachShadow({mode: 'closed'});
+  shadow.innerHTML = '<div style="margin: 2px;">not hit</div>';
+  let aboveHost = document.createElement("div");
+  aboveHost.appendChild(host);
+  container.appendChild(aboveHost);
+  document.documentElement.style = 'background-attachment: scroll; height: 2px;';
+  let boundingRect = host.getBoundingClientRect();
+  assert_array_equals(document.elementsFromPoint(boundingRect.x, boundingRect.y), [host, aboveHost, container, document.body, document.documentElement]);
+  assert_array_equals(shadow.elementsFromPoint(boundingRect.x, boundingRect.y), [host, aboveHost, container, document.body, document.documentElement]);
+}, 'shadowRoot.elementsFromPoint must behave the same with document.elementsFromPoint regarding HTML element');
+
 container.innerHTML = '';
 
     </script>