Add an interface to get URL of isolated entries.

To add a private API to resolve isoalted entries to entries in the external file
system, I need an interface of WebDomFileSystem to obtain URL of isolated
entries, whose the toURL method is invalidated.

BUG=358698
TEST=None

Review URL: https://codereview.chromium.org/279103003

git-svn-id: svn://svn.chromium.org/blink/trunk@174008 bbb929c8-8fbe-4397-9dbb-9b2b20218538