Bug 1919537 - don't crash with DLP on when drag/drop text to a site on Deny list r=dlp-reviewers,reusable-components-reviewers,handyman,mstriemer

This prevents a problem where Content Analysis responds synchronously (because
the site is on the deny list), which causes a modal window to pop up, which
cancels the drag session. This causes MOZ_ASSERTs() where we expect the drag
session to still be active.

Differential Revision: https://phabricator.services.mozilla.com/D222668

toolkit/content/widgets/browser-custom-element.js

@@ -193,6 +193,10 @@
 
             // Create a request for each text and file item in the DataTransfer
             try {
+              // Tell browser to record the event target and to delay EndDragSession
+              // until the content analysis results are given.
+              dragSession.sendStoreDropTargetAndDelayEndDragSession(event);
+
               // Submit a content analysis request for each checkable entry in the
               // DataTransfer and stop dispatching this drop event.  Reissue the
               // drop if all requests are permitted.
@@ -251,13 +255,12 @@
 
               if (!caPromises.length) {
                 // Nothing was analyzable.
+                dragSession.sendDispatchToDropTargetAndResumeEndDragSession(
+                  true
+                );
                 return;
               }
 
-              // Tell browser to record the event target and to delay EndDragSession
-              // until the results are given.
-              dragSession.sendStoreDropTargetAndDelayEndDragSession(event);
-
               // Only permit the drop if all requests were approved.  Issue dragexit
               // instead of drop if CA rejected the content or there was an error.
               Promise.all(caPromises).then(