Remove usage of self from 2 different threads

Bug:789122

Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Id0417fb700bc955013ca8ab3a1ad2ff7ce82ed54
Reviewed-on: https://chromium-review.googlesource.com/898986
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534045}

ios/chrome/browser/ui/settings/password_exporter.mm

@@ -267,12 +267,14 @@ enum class ReauthenticationStatus {
 
 - (void)deleteTemporaryFile:(NSURL*)passwordsTempFileURL {
   __weak PasswordExporter* weakSelf = self;
-  base::PostTaskWithTraits(
+  base::PostTaskWithTraitsAndReply(
       FROM_HERE, {base::MayBlock(), base::TaskPriority::BACKGROUND},
       base::BindBlockArc(^() {
         base::AssertBlockingAllowed();
         NSFileManager* fileManager = [NSFileManager defaultManager];
         [fileManager removeItemAtURL:passwordsTempFileURL error:nil];
+      }),
+      base::BindBlockArc(^() {
         [weakSelf resetExportState];
       }));
 }