Use correctly-typed error codes for fx_codec_jpx_unittest

Needed to fix the failed pdfium_unittests on windows.  Ironically,
I created these constants so I wouldn't mix up size_t's and off_t's
in these tests, but I didn't apply them consistently.

R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1036743005
1 file changed