[BlinkGenPropertyTrees] Re-add element id failure expectation

The fixed-position-in-fixed-overflow.html web test can still
occasionally fail, for example due to scheduling changes in
https://crrev.com/c/1477260.

This reverts part of commit 0353702a316445d8da0efd9db7a75c7534dd96c0.

Bug: 913170
Change-Id: Id2ec607a4bdbbd3df4f35f202430416e839fb971
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1527540
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641847}
1 file changed