Improves typedefs for display layout related types

This change cleans up type definitions in display_layout.js for
improved readability.

Bug: 1077365
Change-Id: Ib1aac690ca56db9a833cbc7c87985eb80922fc0f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2596063
Commit-Queue: Nick Czajka <czajka@google.com>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841166}

chrome/browser/resources/settings/chromeos/device_page/display_layout.js

@@ -9,9 +9,15 @@
  */
 
 /**
- * Required memeber fields for events which select displays.
- * @typedef {!{model: !{item: !chrome.system.display.DisplayUnitInfo}, target:
- *     !HTMLDivElement}} DisplaySelectEvent
+ * Container for DisplayUnitInfo.  Mostly here to make the DisplaySelectEvent
+ * typedef more readable.
+ * @typedef {{item: !chrome.system.display.DisplayUnitInfo}}
+ */
+let InfoItem;
+
+/**
+ * Required member fields for events which select displays.
+ * @typedef {{model: !InfoItem, target: !HTMLDivElement}}
  */
 let DisplaySelectEvent;