Bug 1816740 [3/6] - convert nsIFilePicker::result* into a proper enum r=ipc-reviewers,karlt,mstange,handyman,nika

Convert the various result* constants in nsIFilePicker into a proper
enum, and perform validation when passing it across IPC.

Differential Revision: https://phabricator.services.mozilla.com/D169852

dom/html/HTMLInputElement.cpp

@@ -443,7 +443,8 @@ void GetDOMFileOrDirectoryPath(const OwningFileOrDirectory& aData,
 }  // namespace
 
 NS_IMETHODIMP
-HTMLInputElement::nsFilePickerShownCallback::Done(int16_t aResult) {
+HTMLInputElement::nsFilePickerShownCallback::Done(
+    nsIFilePicker::ResultCode aResult) {
   mInput->PickerClosed();
 
   if (aResult == nsIFilePicker::returnCancel) {