[ElementTiming] Add Element's elementTiming attribute getter

Relevant Intent to Ship:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/DXSoS7dMmok

Bug: 879270, 942033
Change-Id: I399489e766f4eafdd3087f159320dc3211bf1c01
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715257
Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681013}

element-timing/resources/element-timing-helpers.js

@@ -9,8 +9,11 @@ function checkElementInternal(entry, expectedUrl, expectedIdentifier, expectedID
   assert_equals(entry.id, expectedID);
   assert_greater_than_equal(entry.renderTime, beforeRender);
   assert_greater_than_equal(performance.now(), entry.renderTime);
-  if (expectedElement !== null)
+  if (expectedElement !== null) {
     assert_equals(entry.element, expectedElement);
+    assert_equals(entry.identifier, expectedElement.elementTiming);
+    assert_equals(entry.id, expectedElement.id);
+  }
 }
 
 // Checks that this is an ElementTiming entry with url |expectedUrl|. It also