[net] Better StopCaching() handling for HttpCache::Transaction.

net/url_request/url_request.h

Index: net/url_request/url_request.h
diff --git a/net/url_request/url_request.h b/net/url_request/url_request.h
index f9b109e1a241d3af9c4582fde3bfc5da3ac96e16..1cfb6c11c6e1f67412f71167a9cd78d6f695510a 100644
--- a/net/url_request/url_request.h
+++ b/net/url_request/url_request.h
@@ -550,6 +550,8 @@ class NET_EXPORT URLRequest : NON_EXPORTED_BASE(public base::NonThreadSafe),
   // for the same resource. The typical example is a request that results in
   // the data being stored to disk (downloaded instead of rendered) so we don't
   // want to store it twice.
+  //
+  // Should only be called if there are no pending operations.
   void StopCaching();
 
   // This method may be called to follow a redirect that was deferred in