Reland "Add IgnoreDuplicateNavs to fieldtrial_testing_config.json"

The IgnoreDuplicateNavsOnlyWithUserGesture flag is added because the
IgnoreDuplicateNavs logic breaks tests that perform rapid programmatic
navigations, such as setting an iframe's srcdoc immediately after
creation: https://screenshot.googleplex.com/7eLsZ4sj6JNsc8V

The plan is to enable IgnoreDuplicateNavsOnlyWithUserGesture flag only
in tests for now, and then run a experiment to see the impact after the
main kIgnoreDuplicateNavs feature has launched.

While some failures were observed in MegaCQ, they don't seem to be
caused by this change.

This is a reland of commit 2b4d02a8b839fec8b8cefa4c4dc159ceafc4782b

Original change's description:
> Add IgnoreDuplicateNavs to fieldtrial_testing_config.json
>
> Bug: 366060351
> Change-Id: I0568e814033cf8d49bf8b2aa0ceba263d33ab6c8
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6682960
> Commit-Queue: Anna Sato <annasato@chromium.org>
> Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
> Reviewed-by: Mingyu Lei <leimy@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1486820}

Bug: 366060351
Change-Id: Ic08e13bfb1eb61deb611193775d7c595ec6e515b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6853243
Commit-Queue: Anna Sato <annasato@chromium.org>
Reviewed-by: Fergal Daly <fergal@chromium.org>
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1512236}