Remove ResourceFetcher's access to a real FetchContext at frame detach time

Source/core/loader/FrameFetchContext.h

Index: Source/core/loader/FrameFetchContext.h
diff --git a/Source/core/loader/FrameFetchContext.h b/Source/core/loader/FrameFetchContext.h
index 9b27d710da0055c0329c8b7e0592c17423451050..c3f4d66962cdb0cfa977cbe0c2c7b73c13c50bee 100644
--- a/Source/core/loader/FrameFetchContext.h
+++ b/Source/core/loader/FrameFetchContext.h
@@ -54,11 +54,16 @@ public:
         return ResourceFetcher::create(adoptPtrWillBeNoop(new FrameFetchContext(loader)));
     }
 
+    static void provideDocumentToContext(FetchContext& context, Document* document)
+    {
+        ASSERT(document);
+        RELEASE_ASSERT(context.isLiveContext());
+        static_cast<FrameFetchContext&>(context).m_document = document;
+    }
+
     ~FrameFetchContext();
 
-    Document* document() const { return m_document; } // Can be null
-    void setDocument(Document* document) { m_document = document; }
-    void clearDocumentLoader() { m_documentLoader = nullptr; }
+    bool isLiveContext() { return true; }
 
     void addAdditionalRequestHeaders(ResourceRequest&, FetchResourceType) override;
     void setFirstPartyForCookies(ResourceRequest&) override;