Bug 1503832 - Update spellchecker code to deal with Shadow DOM; r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D10579

--HG--
extra : moz-landing-system : lando

toolkit/modules/InlineSpellCheckerContent.jsm

@@ -27,7 +27,7 @@ var InlineSpellCheckerContent = {
     } else {
       // Use the element's editor.
       spellChecker = this._spellChecker =
-        new InlineSpellChecker(event.target.editor);
+        new InlineSpellChecker(event.composedTarget.editor);
     }
 
     this._spellChecker.initFromEvent(event.rangeParent, event.rangeOffset);