[ios] Fix Translate Modal Change Language Crash

When attempting to get the source languages to switch to,
a crash can occur if the self.newSourceLanguageIndex is -1.
In this line of code, it should be self.newTargetLanguageIndex

Bug: 1113818
Change-Id: I4260bc32ba620927a60a966ee8b67c4b1586bc02
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2341388
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Auto-Submit: Chris Lu <thegreenfrog@chromium.org>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795576}

ios/chrome/browser/ui/overlays/infobar_modal/translate/translate_infobar_modal_overlay_mediator.mm

@@ -282,7 +282,7 @@ using translate_infobar_overlays::TranslateModalRequestConfig;
   // check or not show.
   base::string16 targetLanguageName =
       self.newTargetLanguageIndex != kInvalidLanguageIndex
-          ? self.config->language_names().at(self.newSourceLanguageIndex)
+          ? self.config->language_names().at(self.newTargetLanguageIndex)
           : self.config->target_language_name();
 
   NSMutableArray<TableViewTextItem*>* items = [NSMutableArray array];