Remove FX_BSTR and FX_WSTR typedefs.

These stand for const CFX_{Byte,Wide}StringC&, which is just
monumentally confusing, since there are so many string types
running around here.

The following had manual changes:
core/src/fpdfapi/fpdf_edit/fpdf_edit_content.cpp
core/src/fpdfapi/fpdf_render/fpdf_render_loadimage.cpp
core/src/fpdfdoc/doc_form.cpp
fpdfsdk/src/fpdf_ext.cpp

R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1180593004.