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In particular:
* Fix conversion of UNIX file name "/////file" and "////" as a UNIX
  path.  Both of these were broken, due to the same bug.
* Remove a DCHECK that would trigger on "//" as a file name. It's a
  valid UNIX path, if not a file name, and can still be passed through
  the file name conversion code.  Since there's no code to prevent
  that from happening, and the conversion code handles "/", the DCHECK
  made no sense.

BUG=675111

Review-Url: https://codereview.chromium.org/2610973002
Cr-Commit-Position: refs/heads/master@{#441423}