Replaces serializeNodeAsJSONToOutputStream() by getBookmarksTree()
Replaces serializeNodeAsJSONToOutputStream() by getBookmarksTree()

PlacesUtils.serializeNodeAsJSONToOutputStream() was deprecated from Firefox30.
Once it seemed to be moved to BookmarkJSONUtils, but finally removed.

This replaces it by PlacesBackups.getBookmarksTree().
It is asynchronous function, then this makes show_bookmarks() asynchronous
processing.

alprut/bkviewer