[heap] Fix typo in concurrent marker ifdef.

BUG=chromium:694255

Change-Id: Ifb4f72671fd47f0f11118302617b13695bb4407c
Reviewed-on: https://chromium-review.googlesource.com/563376Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46470}

src/heap/incremental-marking.cc

@@ -238,7 +238,7 @@ class IncrementalMarkingMarkingVisitor final
       int start_offset =
           Max(FixedArray::BodyDescriptor::kStartOffset, chunk->progress_bar());
       if (start_offset < object_size) {
-#ifdef CONCURRENT_MARKING
+#ifdef V8_CONCURRENT_MARKING
         incremental_marking_->marking_worklist()->PushBailout(object);
 #else
         if (ObjectMarking::IsGrey<IncrementalMarking::kAtomicity>(