[heap] Add a flag guard for concurrent store buffer processing.

BUG=v8:5580

Review-Url: https://codereview.chromium.org/2460873003
Cr-Commit-Position: refs/heads/master@{#41095}

src/heap/store-buffer.cc

@@ -85,7 +85,7 @@ void StoreBuffer::FlipStoreBuffers() {
   current_ = other;
   top_ = start_[current_];
 
-  if (!task_running_) {
+  if (!task_running_ && FLAG_concurrent_sweeping) {
     task_running_ = true;
     Task* task = new Task(heap_->isolate(), this);
     V8::GetCurrentPlatform()->CallOnBackgroundThread(