XFA: merge patch from CL 733693003, getting rid of more casts

XFA: merge patch from CL 733693003, getting rid of more casts

Getting rid of more (FX_LPCWSTR) casts and fixing two bugs revealed by this.

Since casts to FX_LPCWSTR have been shown to hide bugs I tried removing
more of them, targeting those places where a cast was used to force a
conversion from CFX_WideString to FX_LPCWSTR, replacing these casts with
calls to the newly added .c_str() function. This revealed two places
where the cast was hiding a bug -- where ->c_str() was required instead!

This removes ~33 FX_LPCWSTR casts and there are ~31 left, many of which
will go away in some future change.

Also includes this change:

Removing unnecessary casts from wchar_t* to wchar_t*, by various names.

Original patch from Bruce Dawson(brucedawson@chromium.org)

TBR=bo_xu@foxitsoftware.com, tsepez@chromium.org

Committed: https://pdfium.googlesource.com/pdfium/+/0a476d22c89eb99d36dfd6c9955dd12d1513a870

[brucedawson, 2014-12-23 20:21:07]

Committed patchset #1 (id:1) manually as
0a476d22c89eb99d36dfd6c9955dd12d1513a870 (presubmit successful).