Bug 783846 - Use SelectionHandler._getEditor when moving caret. r=wesj

mobile/android/chrome/content/SelectionHandler.js

@@ -965,7 +965,7 @@ var SelectionHandler = {
   _moveCaret: function sh_moveCaret(aX, aY) {
     // Get rect of text inside element
     let range = document.createRange();
-    range.selectNodeContents(this._targetElement.QueryInterface(Ci.nsIDOMNSEditableElement).editor.rootElement);
+    range.selectNodeContents(this._getEditor().rootElement);
     let textBounds = range.getBoundingClientRect();
 
     // Get rect of editor