Add fieldtrial entry for WebViewClearFunctorInBackground.

Bug: 1424346
Change-Id: Ib1fe6735efe7915fd3b33309e058ad797ad4e64e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4754451


Reviewed-by: Anthony Vallée-Dubois <anthonyvd@chromium.org>
Commit-Queue: Thiabaud Engelbrecht <thiabaud@google.com>
Cr-Commit-Position: refs/heads/main@{#1180454}