[regexp] do not assume short external strings have a minimum size.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index c1e08a2d78eb1faa8073860e1484534f3071fb2f..b0fb264e7e30dad69191a285e25786f664f2ad5e 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -9231,9 +9231,6 @@ class ExternalString: public String {
   static const int kResourceDataOffset = kResourceOffset + kPointerSize;
   static const int kSize = kResourceDataOffset + kPointerSize;
 
-  static const int kMaxShortLength =
-      (kShortSize - SeqString::kHeaderSize) / kCharSize;
-
   // Return whether external string is short (data pointer is not cached).
   inline bool is_short();