Revert "[heap] Disable parallel scavenge."

This reverts commit feb3ee40.

Reason for revert: canary crashes were caused by another CL.

Original change's description:
> [heap] Disable parallel scavenge.
> 
> Bug: chromium:752750
> TBR: mlippautz@chromium.org
> Change-Id: I95ca30742c05be03dcaebb0cd32af04b226e23e2
> Reviewed-on: https://chromium-review.googlesource.com/603652
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#47201}

TBR=ulan@chromium.org,mlippautz@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: chromium:752750
Change-Id: Id26e58ded32af364b3274d6d1a7215944386bb70
Reviewed-on: https://chromium-review.googlesource.com/607891Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47244}

src/flag-definitions.h

@@ -612,7 +612,7 @@ DEFINE_BOOL(age_code, true, "track un-executed functions to age code")
 DEFINE_BOOL(incremental_marking, true, "use incremental marking")
 DEFINE_BOOL(incremental_marking_wrappers, true,
             "use incremental marking for marking wrappers")
-DEFINE_BOOL(parallel_scavenge, false, "parallel scavenge")
+DEFINE_BOOL(parallel_scavenge, true, "parallel scavenge")
 DEFINE_BOOL(trace_parallel_scavenge, false, "trace parallel scavenge")
 #ifdef V8_CONCURRENT_MARKING
 #define V8_CONCURRENT_MARKING_BOOL true