[Tizen][Desktop] WebApp is able to access files via file:// protocol

This change allows an application to access files via file:// protocol
if its manifest contains 'csp' field and the applied policy allows
fetching from 'localhost' (e.g. "csp": "default-src file://localhost").

The application should use the URLs in the following format:
file://localhost/path/to/the/local/file

BUG=XWALK-1840
There is a difference between navigating to an URL and loading an URL as a subresource. I guess we only want the latter here. Not really sure how they are supposed to be combined @anssik