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

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 de63ed39f30123eb4f7bdedc81a2074e938e9dd2..7d849b87e734408009dd5ed6939fe76011ed4264 100644
--- a/Source/core/fetch/FetchContext.h
+++ b/Source/core/fetch/FetchContext.h
@@ -56,14 +56,13 @@ enum FetchResourceType {
 class 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;