[heap] Fix incremental wrapper tracing toggle

The flag is always on and support for turning it off is broken with
conservative barriers.

Bug: 
Change-Id: I1ff548f95d220bf0fcb6df7a1bf5f8a342163696
Reviewed-on: https://chromium-review.googlesource.com/624494
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47482}

src/heap/incremental-marking.cc

@@ -536,7 +536,7 @@ void IncrementalMarking::StartMarking() {
 
   SetState(MARKING);
 
-  {
+  if (FLAG_incremental_marking_wrappers) {
     TRACE_GC(heap()->tracer(),
              GCTracer::Scope::MC_INCREMENTAL_WRAPPER_PROLOGUE);
     heap_->local_embedder_heap_tracer()->TracePrologue();