gn: Reuse IsAsciiAlpha() function from base.

tools/gn/tokenizer.h

Index: tools/gn/tokenizer.h
diff --git a/tools/gn/tokenizer.h b/tools/gn/tokenizer.h
index a980478729aecfa4f2622fdc546f8bdabc495533..13d79097ce39a1f38809f2f53c122680896d5b77 100644
--- a/tools/gn/tokenizer.h
+++ b/tools/gn/tokenizer.h
@@ -33,7 +33,7 @@ class Tokenizer {
   static bool IsNewline(const base::StringPiece& buffer, size_t offset);
 
   static bool IsIdentifierFirstChar(char c) {
-    return (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z') || c == '_';
+    return IsAsciiAlpha(c) || c == '_';
   }
 
   static bool IsIdentifierContinuingChar(char c) {