FileURLToFilePath: Don't unescape '/' and '\\'.

      GURL leaves these escaped, and unescaping them in paths changes the
meaning of the path.

Added two values to the UnescapeRule enumeration:
PATH_SEPARATORS and URL_SPECIAL_CHARS_EXCEPT_PATH_SEPARATORS.

In followup CLs, I intend to replace all uses of URL_SPECIAL_CHARS,
in favor of one or both the two new values, and eventually remove
the value, as it's easily to use in an unsafe manner.

BUG=586657

Review URL: https://codereview.chromium.org/1704163003

Cr-Commit-Position: refs/heads/master@{#377013}