gn flag: v8_enable_minor_mc

The default is 'true' until infra is updated to be able to build and
test with it using this flag.

Bug: v8:7638
Change-Id: I7946eb9bf4087c528d1a844b156a726a1c0671bf
Reviewed-on: https://chromium-review.googlesource.com/1000777
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52485}