Refactor RegExpStub to check lazily.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 1dc3865fd6009b6bf15e901c4b75cbabb1ca3f9a..1d5517b4f47725a70d68ae6182e4b8178ea40ffc 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -7721,6 +7721,9 @@ 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();