Change type of path member of FaviconDatabasePrivate class

Change type of path member of FaviconDatabasePrivate class

path member of FaviconDatabasePrivate type changed from
std::string to base::FilePath making it less error-prone.
Proper checks has been added for that type.

Original beta/m42_2311_t patches:
- http://165.213.202.130/gerrit/#/c/85274/
- http://165.213.202.130/gerrit/#/c/85275/

Bug: http://107.108.218.239/bugzilla/show_bug.cgi?id=14785

Reviewed by: d.waslicki, m.roj, msu.koo

Change-Id: Iec556b76f6eaade557b07d55fc42651c3f0d7f93
Signed-off-by: Marcin Niesluchowski <m.niesluchow@samsung.com>