WTF: Remove kQuestionMarksForUnencodables as its final real user is gone.

Bug: 661819
Change-Id: Id393c7dca3652884287d328c9aa76a4044b00f85
Reviewed-on: https://chromium-review.googlesource.com/780580


Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Joshua Bell <jsbell@chromium.org>
Commit-Queue: Benjamin Wiley Sittler <bsittler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518303}