Introduce ThreadableLoadingContext: make threaded loading code one step away from Document

third_party/WebKit/Source/core/loader/WorkerThreadableLoader.h

Index: third_party/WebKit/Source/core/loader/WorkerThreadableLoader.h
diff --git a/third_party/WebKit/Source/core/loader/WorkerThreadableLoader.h b/third_party/WebKit/Source/core/loader/WorkerThreadableLoader.h
index 6eabba2160269025e26e2de33ff1bb31aee89138..a942efc8054e144d1a04cf748ab422717256eb43 100644
--- a/third_party/WebKit/Source/core/loader/WorkerThreadableLoader.h
+++ b/third_party/WebKit/Source/core/loader/WorkerThreadableLoader.h
@@ -47,6 +47,7 @@
 
 namespace blink {
 
+class LoadingContext;
 class ResourceError;
 class ResourceRequest;
 class ResourceResponse;
@@ -172,7 +173,7 @@ class WorkerThreadableLoader final : public ThreadableLoader {
 
    private:
     MainThreadLoaderHolder(TaskForwarder*, WorkerThreadLifecycleContext*);
-    void start(Document&,
+    void start(LoadingContext&,
                std::unique_ptr<CrossThreadResourceRequestData>,
                const ThreadableLoaderOptions&,
                const ResourceLoaderOptions&);