Factor out gin::IsolateHolder's constructor

third_party/WebKit/Source/core/workers/WorkerBackingThread.cpp

Index: third_party/WebKit/Source/core/workers/WorkerBackingThread.cpp
diff --git a/third_party/WebKit/Source/core/workers/WorkerBackingThread.cpp b/third_party/WebKit/Source/core/workers/WorkerBackingThread.cpp
index 624747ee0b974c1efc6280a9b6b0cbc8278ba8fe..d5a8ac180922e535452df1ec1f546c39b54d2393 100644
--- a/third_party/WebKit/Source/core/workers/WorkerBackingThread.cpp
+++ b/third_party/WebKit/Source/core/workers/WorkerBackingThread.cpp
@@ -61,8 +61,10 @@ WorkerBackingThread::~WorkerBackingThread() {}
 void WorkerBackingThread::initialize() {
   DCHECK(!m_isolate);
   m_backingThread->initialize();
-  m_isolate = V8PerIsolateData::initialize(
-      m_backingThread->platformThread().getWebTaskRunner());
+  WebTaskRunner* taskRunner =
+      backingThread().platformThread().getWebTaskRunner();
+  m_isolate = V8PerIsolateData::initialize(WTF::makeUnique<gin::IsolateHolder>(
+      taskRunner ? taskRunner->toSingleThreadTaskRunner() : nullptr));
   addWorkerIsolate(m_isolate);
   V8Initializer::initializeWorker(m_isolate);