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

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

Index: third_party/WebKit/Source/core/loader/DocumentThreadableLoader.h
diff --git a/third_party/WebKit/Source/core/loader/DocumentThreadableLoader.h b/third_party/WebKit/Source/core/loader/DocumentThreadableLoader.h
index 7303b3eed99b2cbf6541b82ba89e5a7dc13b4bda..dde41e3b98e15088bbface43ac1894d664ef3977 100644
--- a/third_party/WebKit/Source/core/loader/DocumentThreadableLoader.h
+++ b/third_party/WebKit/Source/core/loader/DocumentThreadableLoader.h
@@ -49,6 +49,7 @@ namespace blink {
 
 class Document;
 class KURL;
+class LoadingContext;
 class ResourceRequest;
 class SecurityOrigin;
 class ThreadableLoaderClient;
@@ -67,6 +68,10 @@ class CORE_EXPORT DocumentThreadableLoader final : public ThreadableLoader,
                                           ThreadableLoaderClient*,
                                           const ThreadableLoaderOptions&,
                                           const ResourceLoaderOptions&);
+  static DocumentThreadableLoader* create(LoadingContext&,
+                                          ThreadableLoaderClient*,
+                                          const ThreadableLoaderOptions&,
+                                          const ResourceLoaderOptions&);
   ~DocumentThreadableLoader() override;
 
   void start(const ResourceRequest&) override;
@@ -81,7 +86,7 @@ class CORE_EXPORT DocumentThreadableLoader final : public ThreadableLoader,
  private:
   enum BlockingBehavior { LoadSynchronously, LoadAsynchronously };
 
-  DocumentThreadableLoader(Document&,
+  DocumentThreadableLoader(LoadingContext&,
                            ThreadableLoaderClient*,
                            BlockingBehavior,
                            const ThreadableLoaderOptions&,
@@ -182,10 +187,12 @@ class CORE_EXPORT DocumentThreadableLoader final : public ThreadableLoader,
   // End of ResourceOwner re-implementation, see above.
 
   const SecurityOrigin* getSecurityOrigin() const;
-  Document& document() const;
+
+  // TODO(kinuko): Remove dependency to document.
+  Document* document() const;
 
   ThreadableLoaderClient* m_client;
-  Member<Document> m_document;
+  Member<LoadingContext> m_loadingContext;
 
   const ThreadableLoaderOptions m_options;
   // Some items may be overridden by m_forceDoNotAllowStoredCredentials and