Work-around for filesystem:-URL bug (#4592).

Visit https://robwu.nl/pdfjs/filesystem-worker-bug.htm and follow the
steps to verify that the extension works as intended with this patch.
Move logic from viewer.js to chromecom.js

and also default to '' instead of DEFAULT_URL to avoid trying to
load a non-existent file when no file has been specified.
Merge pull request #4598 from Rob--W/crx-filesystem-worker

Work-around for filesystem:-URL bug