Prefer {byte,wide}string.h inclusion over fx_string.h

Most files only care to operate on one string type, so we can halve the
inclusions that fx_string.h would provide.

Change-Id: I1dcbb008af9ee31c9151051b0505cb62cd451802
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/87310
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
diff --git a/xfa/fxfa/cxfa_ffapp.h b/xfa/fxfa/cxfa_ffapp.h
index d26b90c..fe41809 100644
--- a/xfa/fxfa/cxfa_ffapp.h
+++ b/xfa/fxfa/cxfa_ffapp.h
@@ -7,8 +7,8 @@
 #ifndef XFA_FXFA_CXFA_FFAPP_H_
 #define XFA_FXFA_CXFA_FFAPP_H_
 
-#include "core/fxcrt/fx_string.h"
 #include "core/fxcrt/unowned_ptr.h"
+#include "core/fxcrt/widestring.h"
 #include "fxjs/gc/heap.h"
 #include "v8/include/cppgc/garbage-collected.h"
 #include "v8/include/cppgc/member.h"