Ensure that the worker pool is running if we intend to use it

src/v8.cc

Index: src/v8.cc
diff --git a/src/v8.cc b/src/v8.cc
index 28454b437e5d9f75141572e37865b01e3220dfae..a415f26197b7f5cf9d001823c39021bef4d7703c 100644
--- a/src/v8.cc
+++ b/src/v8.cc
@@ -82,6 +82,8 @@ bool V8::Initialize(Deserializer* des) {
 #ifdef V8_USE_DEFAULT_PLATFORM
   DefaultPlatform* platform = static_cast<DefaultPlatform*>(platform_);
   platform->SetThreadPoolSize(isolate->max_available_threads());
+  // We currently only start the threads early, if we know that we'll use them.
+  if (FLAG_job_based_sweeping) platform->EnsureInitialized();
 #endif
 
   return isolate->Init(des);