Fix and re-enable URLRequestFTP tests (Except on Android).

Fix and re-enable URLRequestFTP tests (Except on Android).

The tests were disabled because of two failures:

* The test server was failing to start.  There were some CLs to fix this
landed quite a while back, and the fact that
URLRequestTestFTP.UnfsafePort, which (needlessly) started the FTP server
was still passing indicates that this isn't a problem any longer.

* Tests were having trouble accessing some test files.  This CL modifies
the test to use a file in the url_request_unittest directory, like the
other URLRequestTest subclasses, which should hopefully fix this.

Additionally, FTP is no longer enabled in the TestURLRequestContext by
default, so this CL also sets up FTP support in the URLRequestTestFTP
test fixture.

BUG=25045,495220

Review URL: https://codereview.chromium.org/1160273003

Cr-Commit-Position: refs/heads/master@{#332266}