When a top level browsing context that has a file:// URI page loaded is navigated to a URI that would not normally be loaded in the file content process and it is the only top level in its tab group, then it should be safe (non-web observable) to process switch to a normal content process. We want to do this as the normal content process will have a more secure sandbox. Currently POST data is lost from history entries on process switch, so there is at least some observable behaviour change.