Use #if DCHECK_IS_ON() in base/strings/string_piece.*

base/strings/string_piece.cc

Index: base/strings/string_piece.cc
diff --git a/base/strings/string_piece.cc b/base/strings/string_piece.cc
index 99975725afcc1d7d5dc104b076be55e674d031b1..db0433aed152462c21ce63c3053d79038b14eb96 100644
--- a/base/strings/string_piece.cc
+++ b/base/strings/string_piece.cc
@@ -435,7 +435,7 @@ StringPiece16 substr(const StringPiece16& self,
   return substrT(self, pos, n);
 }
 
-#if !defined(NDEBUG) || defined(DCHECK_ALWAYS_ON)
+#if DCHECK_IS_ON()
 void AssertIteratorsInOrder(std::string::const_iterator begin,
                             std::string::const_iterator end) {
   DCHECK(begin <= end) << "StringPiece iterators swapped or invalid.";