Revert of [heap] Turn on parallel compaction (patchset #1 id:1 of...

Revert of [heap] Turn on parallel compaction (patchset #1 id:1 of https://codereview.chromium.org/1364693002/ )

Reason for revert:
Fails on gc stress
  https://chromegw.corp.google.com/i/client.v8/builders/V8%20Linux%20-%20gc%20stress/builds/157/

Original issue's description:
> [heap] Turn on parallel compaction
>
> R=hpayer@chromium.org
> BUG=chromium:524425
> LOG=N
>
> Committed: https://crrev.com/04db5bfa915766b228218ddc748af308b57ae8ea
> Cr-Commit-Position: refs/heads/master@{#31763}

TBR=hpayer@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=chromium:524425

Review URL: https://codereview.chromium.org/1424313008

Cr-Commit-Position: refs/heads/master@{#31765}

src/flag-definitions.h

@@ -673,7 +673,7 @@ 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, true, "use parallel compaction")
+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,