[heap] Disable parallel scavenge for single-threaded use

Bug: chromium:738865
Change-Id: I7c367116e60117eab36d58d5078a87841e4b03cc
Reviewed-on: https://chromium-review.googlesource.com/584833Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46872}

src/flag-definitions.h

@@ -1271,6 +1271,7 @@ DEFINE_NEG_IMPLICATION(single_threaded, concurrent_sweeping)
 DEFINE_NEG_IMPLICATION(single_threaded, minor_mc_parallel_marking)
 DEFINE_NEG_IMPLICATION(single_threaded, parallel_compaction)
 DEFINE_NEG_IMPLICATION(single_threaded, parallel_pointer_update)
+DEFINE_NEG_IMPLICATION(single_threaded, parallel_scavenge)
 DEFINE_NEG_IMPLICATION(single_threaded, concurrent_store_buffer)
 DEFINE_NEG_IMPLICATION(single_threaded, compiler_dispatcher)