[heap] Promote young objects by default in MC

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}

src/flags/flag-definitions.h

@@ -355,7 +355,7 @@ DEFINE_BOOL(assert_types, false,
 DEFINE_BOOL(allocation_site_pretenuring, true,
             "pretenure with allocation sites")
 DEFINE_BOOL(page_promotion, true, "promote pages based on utilization")
-DEFINE_BOOL(always_promote_young_mc, false,
+DEFINE_BOOL(always_promote_young_mc, true,
             "always promote young objects during mark-compact")
 DEFINE_INT(page_promotion_threshold, 70,
            "min percentage of live bytes on a page to enable fast evacuation")