[heap] Implication for parallel ephemeron flags

Disable parallel ephemeron handling if single-threaded flag is set.

Bug: chromium:844008
Change-Id: I96d3daae4a239a5326c6d3394f620697bad1780a
Reviewed-on: https://chromium-review.googlesource.com/1102335Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@google.com>
Cr-Commit-Position: refs/heads/master@{#53761}

src/flag-definitions.h

@@ -1358,6 +1358,8 @@ DEFINE_NEG_IMPLICATION(single_threaded_gc, parallel_compaction)
 DEFINE_NEG_IMPLICATION(single_threaded_gc, parallel_marking)
 DEFINE_NEG_IMPLICATION(single_threaded_gc, parallel_pointer_update)
 DEFINE_NEG_IMPLICATION(single_threaded_gc, parallel_scavenge)
+DEFINE_NEG_IMPLICATION(single_threaded_gc, parallel_ephemeron_marking)
+DEFINE_NEG_IMPLICATION(single_threaded_gc, parallel_ephemeron_visiting)
 DEFINE_NEG_IMPLICATION(single_threaded_gc, concurrent_store_buffer)
 #ifdef ENABLE_MINOR_MC
 DEFINE_NEG_IMPLICATION(single_threaded_gc, minor_mc_parallel_marking)