Replace base::str[n]casecmp with helper functions.

tools/gn/filesystem_utils.cc

Index: tools/gn/filesystem_utils.cc
diff --git a/tools/gn/filesystem_utils.cc b/tools/gn/filesystem_utils.cc
index c98db86484c1aca2925ae2923e426a9055a81e0b..42736c9fbfa690b74f18fc088d6b013351adac27 100644
--- a/tools/gn/filesystem_utils.cc
+++ b/tools/gn/filesystem_utils.cc
@@ -83,7 +83,7 @@ bool AreAbsoluteWindowsPathsEqual(const base::StringPiece& a,
     return false;
 
   // For now, just do a case-insensitive ASCII comparison. We could convert to
-  // UTF-16 and use ICU if necessary. Or maybe base::strcasecmp is good enough?
+  // UTF-16 and use ICU if necessary.
   for (size_t i = 0; i < a.size(); i++) {
     if (NormalizeWindowsPathChar(a[i]) != NormalizeWindowsPathChar(b[i]))
       return false;