Fix filepath tests for EncodingError

      As the issue on the management of dot-dot-dot path sequences has been
solved, this change re-enables and fixes the filepath  tests and removes
the corresponding FIXMEs.

BUG=181617

Review URL: https://codereview.chromium.org/227093003

git-svn-id: svn://svn.chromium.org/blink/trunk@171330 bbb929c8-8fbe-4397-9dbb-9b2b20218538