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

Committed: https://pdfium.googlesource.com/pdfium/+/ee7412f5aef353e5c6f1a64d0e1708ed926869d9

Committed: https://pdfium.googlesource.com/pdfium/+/5a256ad29483eb2b13e6e2c89fe0f77a9103f68f

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