Run background pages in separate partition

This avoids the default partition always being created on startup, so it
gives users a chance to run session.fromParititon.

lib/browser/chrome-extension.js

@@ -86,6 +86,7 @@ const startBackgroundPages = function (manifest) {
   }
 
   const contents = webContents.create({
+    partition: 'persist:__chrome_extension',
     isBackgroundPage: true,
     commandLineSwitches: ['--background-page']
   })