[heap] Enable parallel scavenge

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Bug: chromium:738865
Change-Id: I202f9c61c13fcacfb8fff37458931d91c6f831e5
Reviewed-on: https://chromium-review.googlesource.com/584835Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46920}

src/flag-definitions.h

@@ -616,7 +616,7 @@ 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