Prevent ThreadableLoaderClient methods from being called after failure notification

Source/core/loader/ThreadableLoader.h

Index: Source/core/loader/ThreadableLoader.h
diff --git a/Source/core/loader/ThreadableLoader.h b/Source/core/loader/ThreadableLoader.h
index dc8f3ef02e9da2a4776cb0e5e284f45cf2d0d75c..44b8fd73dd29f19808b360b7b340545ace314a3c 100644
--- a/Source/core/loader/ThreadableLoader.h
+++ b/Source/core/loader/ThreadableLoader.h
@@ -115,16 +115,25 @@ namespace blink {
 
     // Useful for doing loader operations from any thread (not threadsafe,
     // just able to run on threads other than the main thread).
+    //
+    // Arguments common to both loadResourceSynchronously() and create():
+    //
+    // - ThreadableLoaderOptions argument configures this ThreadableLoader's
+    //   behavior.
+    //
+    // - ResourceLoaderOptions argument will be passed to the FetchRequest
+    //   that this ThreadableLoader creates. It can be altered e.g. when
+    //   redirect happens.
     class CORE_EXPORT ThreadableLoader : public RefCounted<ThreadableLoader> {
         WTF_MAKE_NONCOPYABLE(ThreadableLoader);
     public:
-        // ThreadableLoaderOptions argument configures this ThreadableLoader's
-        // behavior.
-        //
-        // ResourceLoaderOptions argument will be passed to the FetchRequest
-        // that this ThreadableLoader creates. It can be altered e.g. when
-        // redirect happens.
         static void loadResourceSynchronously(ExecutionContext&, const ResourceRequest&, ThreadableLoaderClient&, const ThreadableLoaderOptions&, const ResourceLoaderOptions&);
+        // The following ThreadableLoaderClient methods may destroy the
+        // ThreadableLoader instance in them.
+        // - didFinishLoading()
+        // - didFail()
+        // - didFailAccessControlCheck()
+        // - didFailRedirectCheck()
         static PassRefPtr<ThreadableLoader> create(ExecutionContext&, ThreadableLoaderClient*, const ResourceRequest&, const ThreadableLoaderOptions&, const ResourceLoaderOptions&);
 
         // A ThreadableLoader may have a timeout specified. It is possible, in some cases, for