Remove ResourceFetcher's access to a FrameFetchContext when detaching

Source/core/fetch/FetchContext.h

Index: Source/core/fetch/FetchContext.h
diff --git a/Source/core/fetch/FetchContext.h b/Source/core/fetch/FetchContext.h
index 80135b6e10df9cc691c8b8468c77c0aa2221dbb6..a63e246ba1f3371c438930e412df6aa38d484c90 100644
--- a/Source/core/fetch/FetchContext.h
+++ b/Source/core/fetch/FetchContext.h
@@ -57,14 +57,13 @@ enum FetchResourceType {
 class CORE_EXPORT FetchContext : public NoBaseWillBeGarbageCollectedFinalized<FetchContext> {
     WTF_MAKE_NONCOPYABLE(FetchContext);
 public:
-    static PassOwnPtrWillBeRawPtr<FetchContext> create()
-    {
-        return adoptPtrWillBeNoop(new FetchContext);
-    }
+    static FetchContext& nullInstance();
 
     virtual ~FetchContext() { }
     DEFINE_INLINE_VIRTUAL_TRACE() { }
 
+    virtual bool isLiveContext() { return false; }
+
     virtual void addAdditionalRequestHeaders(ResourceRequest&, FetchResourceType);
     virtual void setFirstPartyForCookies(ResourceRequest&);
     virtual CachePolicy cachePolicy() const;