Merge to XFA: Backfill some FX String/StringC unit tests for == and !=.

Original Review URL: https://codereview.chromium.org/1118973005
Original Review URL: https://codereview.chromium.org/1122573002

R=thestig@chromium.org
TBR=thestig@chromium.org

Review URL: https://codereview.chromium.org/1126433002
3 files changed