I think this is not used & safe to remove: https://cs.chromium.org/search/?q=tough_layout_cases+file:%5Esrc/tools/perf/+package:%5Echromium$&type=cs

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5f1aa08a49586e9241df35440d513393f21883d3

commit 5f1aa08a49586e9241df35440d513393f21883d3
Author: Ned Nguyen <nednguyen@google.com>
Date: Thu Sep 21 22:11:17 2017

Remove references to page_cycler_v2.tough_layout_cases (deleted benchmark)

Bug:  767546 
Change-Id: Ia829774573ba0c329a52055e34be77dfa6a82d52
Reviewed-on: https://chromium-review.googlesource.com/677644
Reviewed-by: Helen Li <xunjieli@chromium.org>
Commit-Queue: Ned Nguyen <nednguyen@google.com>
Cr-Commit-Position: refs/heads/master@{#503579}
[modify] https://crrev.com/5f1aa08a49586e9241df35440d513393f21883d3/tools/perf/core/desktop_benchmark_avg_times.json
[delete] https://crrev.com/98a273dbd384790979baed706eb8907435654314/tools/perf/page_sets/data/tough_layout_cases.json
[delete] https://crrev.com/98a273dbd384790979baed706eb8907435654314/tools/perf/page_sets/data/tough_layout_cases_000.wpr.sha1