Use toDocument instead of static_cast<Document*>.

Source/core/workers/WorkerMessagingProxy.cpp

Index: Source/core/workers/WorkerMessagingProxy.cpp
diff --git a/Source/core/workers/WorkerMessagingProxy.cpp b/Source/core/workers/WorkerMessagingProxy.cpp
index 7af0c0a43e9f4ddb5aa6ca813b5058395d475cdb..db8e2e10de1171385562c056ec31224f5849dc2f 100644
--- a/Source/core/workers/WorkerMessagingProxy.cpp
+++ b/Source/core/workers/WorkerMessagingProxy.cpp
@@ -263,7 +263,7 @@ void WorkerMessagingProxy::startWorkerGlobalScope(const KURL& scriptURL, const S
 {
     // FIXME: This need to be revisited when we support nested worker one day
     ASSERT(m_scriptExecutionContext->isDocument());
-    Document* document = static_cast<Document*>(m_scriptExecutionContext.get());
+    Document* document = toDocument(m_scriptExecutionContext.get());
     RefPtr<DedicatedWorkerThread> thread = DedicatedWorkerThread::create(scriptURL, userAgent, sourceCode, *this, *this, startMode, document->contentSecurityPolicy()->deprecatedHeader(), document->contentSecurityPolicy()->deprecatedHeaderType(), document->topOrigin(), document->loader()->timing()->referenceMonotonicTime());
     workerThreadCreated(thread);
     thread->start();