Reland "[heap] Turn on parallel compaction"

This reverts commit aa4688c1.

BUG=chromium:524425
LOG=N
CQ_EXTRA_TRYBOTS=tryserver.v8:v8_linux_arm64_gc_stress_dbg;tryserver.v8:v8_linux_gc_stress_dbg;tryserver.v8:v8_mac_gc_stress_dbg;tryserver.v8:v8_linux64_msan_rel;tryserver.v8:v8_linux64_tsan_rel

Review URL: https://codereview.chromium.org/1460663002

Cr-Commit-Position: refs/heads/master@{#32088}

src/flag-definitions.h

@@ -674,7 +674,7 @@ DEFINE_INT(min_progress_during_incremental_marking_finalization, 32,
 DEFINE_INT(max_incremental_marking_finalization_rounds, 3,
            "at most try this many times to finalize incremental marking")
 DEFINE_BOOL(concurrent_sweeping, true, "use concurrent sweeping")
-DEFINE_BOOL(parallel_compaction, false, "use parallel compaction")
+DEFINE_BOOL(parallel_compaction, true, "use parallel compaction")
 DEFINE_BOOL(trace_incremental_marking, false,
             "trace progress of the incremental marking")
 DEFINE_BOOL(track_gc_object_stats, false,