[Extension/App Docs]Update to Migration Guide

Removed section referencing Electron.

Bug: none
Change-Id: I37267cf9ca3a70cd004f3f96ae3b5a3b3c7c4af2
Reviewed-on: https://chromium-review.googlesource.com/1012188Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Crystal Lambert <crystallambert@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550664}

chrome/common/extensions/docs/templates/articles/migration.html

@@ -111,13 +111,6 @@ Is this something I can do with a web app?</strong></h3>
 users can add ‘bookmark apps’ to their shelf / desktop,
 which optionally run your web app in its own window.</p>
 
-<h3 id="window-frame"><strong>Q: My app needs to customize the window frame
-(e.g. remove the close button). Can I do this with a web app?</strong></h3>
-
-<p>Window frame customization is not possible with a web app or extension.
-If this is necessary for your application you may want to consider using a tool
-like Electron or NW.js to wrap your application.</p>
-
 <h3 id="shortcuts"><strong>Q: My app supports keyboard shortcuts.
 Can the web support this also?</strong></h3>