Make CFX_WideString::LockBuffer() completely unused.

Then remove CFX_{Wide,Byte}String::LockBuffer().  Prelude to a vast
simplification. There's an additional copy now in one place, so
shoot me.

BUG=pdfium:144
R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1053613004
4 files changed