CSSMin: Change behavior for missing files

We would previously return the path to the local file on the
filesystem, which is useless in all cases and possibly a security
issue in some. Now we return the URL at which the file would be
accessible had it existed.

Also reordered the code around that part to make the control flow
clearer and added a test.

Change-Id: I1d5befb2ea385ae4d316c5d8c5d1fc092b64c4ff
Merge "CSSMin: Change behavior for missing files"