[heap] Allow repeatedly setting EmbedderHeapTracer

Allow swapping the tracer if not in a GC.

BUG=chromium:468240
TBR=ulan@chromium.org
NOTRY=true

Review-Url: https://codereview.chromium.org/2475503004
Cr-Commit-Position: refs/heads/master@{#40719}

src/heap/heap.cc

@@ -5552,8 +5552,7 @@ void Heap::NotifyDeserializationComplete() {
 }
 
 void Heap::SetEmbedderHeapTracer(EmbedderHeapTracer* tracer) {
-  DCHECK_NOT_NULL(tracer);
-  CHECK_NULL(embedder_heap_tracer_);
+  DCHECK_EQ(gc_state_, HeapState::NOT_IN_GC);
   embedder_heap_tracer_ = tracer;
 }