This flag has been on by default for some time. Once
https://chromium-review.googlesource.com/c/v8/v8/+/1270578 lands we need it to
be able to find duplicate parameters (to be spec-compliant).

Change-Id: I222023d7cd955127d3ecca42283b37063e962c58
Reviewed-on: https://chromium-review.googlesource.com/c/1270581
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56468}