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

Source/core/fetch/FetchContext.cpp

Index: Source/core/fetch/FetchContext.cpp
diff --git a/Source/core/fetch/FetchContext.cpp b/Source/core/fetch/FetchContext.cpp
index 62100f93e46799017e5eaf28b72c300cfd1e7eaf..8b5b3ca517934e8d29ca984cb27c74ab78336868 100644
--- a/Source/core/fetch/FetchContext.cpp
+++ b/Source/core/fetch/FetchContext.cpp
@@ -34,6 +34,12 @@
 
 namespace blink {
 
+FetchContext& FetchContext::nullInstance()
+{
+    DEFINE_STATIC_LOCAL(OwnPtrWillBePersistent<FetchContext>, instance, (adoptPtrWillBeNoop(new FetchContext)));
+    return *instance;
+}
+
 void FetchContext::dispatchDidChangeResourcePriority(unsigned long, ResourceLoadPriority, int)
 {
 }