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
Committed: https://crrev.com/9f2ec60cde8ba05ee891bccc84fa1a05bb8d9030
Cr-Commit-Position: refs/heads/master@{#332266}

[mmenke, 2015-06-01 20:33:20]

mmenke@chromium.org changed reviewers:
+ eroman@chromium.org

[mmenke, 2015-06-01 20:33:20]

Minus one fixit bug, plus one more.  The law of conservation of bugs.  :(

[eroman, 2015-06-01 20:46:48]

lgtm

[mmenke, 2015-06-01 20:54:19]

The CQ bit was checked by mmenke@chromium.org

[commit-bot: I haz the power, 2015-06-01 20:54:47]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1160273003/40001

[commit-bot: I haz the power, 2015-06-01 20:59:55]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-06-01 21:00:47]

Patchset 3 (id:??) landed as
https://crrev.com/9f2ec60cde8ba05ee891bccc84fa1a05bb8d9030
Cr-Commit-Position: refs/heads/master@{#332266}