The CL prepares the sources and the tests for enabling
cppgc_enable_young_generation by default. The static initializer
in YoungGenerationEnabler (due to v8::base::Mutex) changed to be lazy.
The tests are now checking the runtime flag.

Bug: chromium:1029379
Change-Id: I1497a3dd2b8d62c1acd48496821f07324b7944d5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3616726Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Auto-Submit: Anton Bikineev <bikineev@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80304}