Start experiment with promoting all young live objects during
mark-compact. This CL sets always_promote_young_mc to true by default.

Change-Id: I306527ad2a361fc3e0edcf684c5ccec2b1d6e491
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1879938Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64630}