[gin] Add V8MaxValidPolymorphicMapCount feature

Add a new feature to lock `max_valid_polymorphic_map_count`
to 10 for an upcoming Finch experiment.

Also update fieldtrial_testing_config.json to enable it in tests.

Bug: 510324467
Change-Id: Icd6d085a4ed4e5461d8426b1510d40f5f20405cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7822620
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Marco Vitale <mrcvtl@chromium.org>
Auto-Submit: Marco Vitale <mrcvtl@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1627652}
NOKEYCHECK=True
GitOrigin-RevId: be6003897421647849c06f03434bb81a7f51e0af