Use chrome.storage.sync to store preferences instead of
chrome.storage.local, to allow settings to be synchronized if the user
chooses to sign in in Chrome and enables synchronization of extension
preferences.
Commit df10513e10 unfortunately broke the options dialog of the Chromium extension because the logic required to work with the preference was not added. This patch adds the required logic to show the preference in the options dialog and to persist it to the preferences storage.
Verified using Chromium 50 on Arch Linux.
