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

Source/core/fetch/ResourceFetcher.h

Index: Source/core/fetch/ResourceFetcher.h
diff --git a/Source/core/fetch/ResourceFetcher.h b/Source/core/fetch/ResourceFetcher.h
index cdc2eebd9144250b26f893a57a5c539e6329cc30..74467714213684b71d57f81fbd30f67ce828b1aa 100644
--- a/Source/core/fetch/ResourceFetcher.h
+++ b/Source/core/fetch/ResourceFetcher.h
@@ -103,7 +103,8 @@ public:
 
     bool shouldDeferImageLoad(const KURL&) const;
 
-    FetchContext& context() const { return *m_context.get(); }
+    FetchContext& context() const { return m_context ? *m_context.get() : FetchContext::nullInstance(); }
+    void clearContext() { m_context.clear(); }
 
     void garbageCollectDocumentResources();