This partially reverts df0836b9b8.
The entry in preferences_schema.json is restored because that is
required to make managed preferences visible to the extension code.
The default key is still removed from default_preferences.json,
because this change only concerns the Chrome extension, not the
other parts of PDF.js. To account for the missing key, the
deprecated key was added back in chromecom.js
The key needs to be restored in preferences_schema.json too,
because that's the only way to make managed preferences visible.
I'm using `Object.assign`, which was introduced in Chrome 45,
so the preference module will break in Chrome 45 and earlier.
This is fine, because we do not support Chrome before 49.
fc83ce1aae
