Crash with aria-activedescendant pointing to <script>

Update code to null check return value of GetOrCreate(). It was
always possible for those methods to return null, if
IsNode/IsLayoutRelevantForAccessibility() is false, or for some
shadow DOM content, but it happens more often now, e.g. for
<style> and <script>.

Bug: 1172833
Change-Id: Ia1a02e9ed042c9e7be3ad81492adcfa286c4776b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2661719


Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: Alice Boxhall <aboxhall@chromium.org>
Auto-Submit: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#849089}