Convert string Find methods to return an Optional

The Find and ReverseFind methods for WideString, WideStringC,
ByteString, and ByteStringC have been converted from returning a raw
FX_STRSIZE, to returning Optional<FX_STRSIZE>, so that success/failure
can be indicated without using FX_STRNPOS.

This allows for removing FX_STRNPOS and by association makes the
conversion of FX_STRSIZE to size_t easier, since it forces checking
the return value of Find to be explictly done as well as taking the
error value out of the range of FX_STRSIZE.

New Contains methods have been added for cases where the success or
failure is all the call site to Find cared about, and the actual
position was ignored.

BUG=pdfium:828


Change-Id: Id827e508c8660affa68cc08a13d96121369364b7
Reviewed-on: https://pdfium-review.googlesource.com/11350
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
Reviewed-by: dsinclair <dsinclair@chromium.org>
diff --git a/fpdfsdk/fpdfedittext.cpp b/fpdfsdk/fpdfedittext.cpp
index 47facb4..38f7cc2 100644
--- a/fpdfsdk/fpdfedittext.cpp
+++ b/fpdfsdk/fpdfedittext.cpp
@@ -39,7 +39,7 @@
   int flags = 0;
   if (FXFT_Is_Face_fixedwidth(pFont->GetFace()))
     flags |= FXFONT_FIXED_PITCH;
-  if (font_name.Find("Serif") != FX_STRNPOS)
+  if (font_name.Contains("Serif"))
     flags |= FXFONT_SERIF;
   if (FXFT_Is_Face_Italic(pFont->GetFace()))
     flags |= FXFONT_ITALIC;