Worker: Add comments about thread restriction to WorkerLoaderProxy

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

Index: third_party/WebKit/Source/core/workers/WorkerLoaderProxy.cpp
diff --git a/third_party/WebKit/Source/core/workers/WorkerLoaderProxy.cpp b/third_party/WebKit/Source/core/workers/WorkerLoaderProxy.cpp
index 7c21c93d3b0d3d3a01aeb5a8d1f1df566ea110bf..dd8655f571c7bfb72956c3472964114519b8b6ad 100644
--- a/third_party/WebKit/Source/core/workers/WorkerLoaderProxy.cpp
+++ b/third_party/WebKit/Source/core/workers/WorkerLoaderProxy.cpp
@@ -19,7 +19,8 @@ WorkerLoaderProxy::~WorkerLoaderProxy() {
 void WorkerLoaderProxy::detachProvider(
     WorkerLoaderProxyProvider* proxyProvider) {
   MutexLocker locker(m_lock);
-  DCHECK(proxyProvider == m_loaderProxyProvider);
+  DCHECK(isMainThread());
+  DCHECK_EQ(proxyProvider, m_loaderProxyProvider);
   m_loaderProxyProvider = nullptr;
 }
 
@@ -27,6 +28,7 @@ void WorkerLoaderProxy::postTaskToLoader(
     const WebTraceLocation& location,
     std::unique_ptr<ExecutionContextTask> task) {
   MutexLocker locker(m_lock);
+  DCHECK(!isMainThread());
   if (!m_loaderProxyProvider)
     return;
   m_loaderProxyProvider->postTaskToLoader(location, std::move(task));
@@ -36,6 +38,7 @@ void WorkerLoaderProxy::postTaskToWorkerGlobalScope(
     const WebTraceLocation& location,
     std::unique_ptr<WTF::CrossThreadClosure> task) {
   MutexLocker locker(m_lock);
+  DCHECK(isMainThread());
   if (!m_loaderProxyProvider)
     return;
   m_loaderProxyProvider->postTaskToWorkerGlobalScope(location, std::move(task));