Add basic support for parallel compaction and flag.

src/flag-definitions.h

Index: src/flag-definitions.h
diff --git a/src/flag-definitions.h b/src/flag-definitions.h
index 51d716e2911f6e868a61c328c69073c1161a6a5b..5e32965844a32efb2fd37c8c99c3e9d9cbaa5fd2 100644
--- a/src/flag-definitions.h
+++ b/src/flag-definitions.h
@@ -659,6 +659,7 @@ DEFINE_INT(min_progress_during_object_groups_marking, 128,
 DEFINE_INT(max_object_groups_marking_rounds, 3,
            "at most try this many times to over approximate the weak closure")
 DEFINE_BOOL(concurrent_sweeping, true, "use concurrent sweeping")
+DEFINE_BOOL(parallel_compaction, false, "use parallel compaction")
 DEFINE_BOOL(trace_incremental_marking, false,
             "trace progress of the incremental marking")
 DEFINE_BOOL(track_gc_object_stats, false,
@@ -808,6 +809,7 @@ DEFINE_BOOL(predictable, false, "enable predictable mode")
 DEFINE_NEG_IMPLICATION(predictable, concurrent_recompilation)
 DEFINE_NEG_IMPLICATION(predictable, concurrent_osr)
 DEFINE_NEG_IMPLICATION(predictable, concurrent_sweeping)
+DEFINE_NEG_IMPLICATION(predictable, parallel_compaction)
 
 // mark-compact.cc
 DEFINE_BOOL(force_marking_deque_overflows, false,