Add FXSYS_IsLowerASCII().

Add FXSYS_IsLowerASCII() to check whether a character is a lowercase
ASCII character.

This CL also adds callers for FXSYS_IsLowerASCII() to replace all
lowercase ASCII character checks across PDFium.

Change-Id: Id131f6c95c5221a69cd9225d421bd58f8e23d02b
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/67210
Commit-Queue: Hui Yingst <nigi@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
diff --git a/fxbarcode/datamatrix/BC_TextEncoder.cpp b/fxbarcode/datamatrix/BC_TextEncoder.cpp
index 10ee1d0..5ed444a 100644
--- a/fxbarcode/datamatrix/BC_TextEncoder.cpp
+++ b/fxbarcode/datamatrix/BC_TextEncoder.cpp
@@ -47,7 +47,7 @@
     *sb += (wchar_t)(c - 48 + 4);
     return 1;
   }
-  if (c >= 'a' && c <= 'z') {
+  if (FXSYS_IsLowerASCII(c)) {
     *sb += (wchar_t)(c - 97 + 14);
     return 1;
   }