[builtins] Move StringStartsWith to a C++ builtin.

src/builtins/builtins.h

Index: src/builtins/builtins.h
diff --git a/src/builtins/builtins.h b/src/builtins/builtins.h
index a8d9f9ce4c624b39e15d361704d0f33e0fb25729..183ece8dac0c3d130b4027fe0404ff47c2f07f3a 100644
--- a/src/builtins/builtins.h
+++ b/src/builtins/builtins.h
@@ -629,6 +629,9 @@ namespace internal {
   TFJ(StringPrototypeSubstr, 3)                                               \
   /* ES6 section 21.1.3.19 String.prototype.substring ( start, end ) */       \
   TFJ(StringPrototypeSubstring, 3)                                            \
+  /* ES6 section 21.1.3.20 */                                                 \
+  /* String.prototype.startsWith ( searchString [ , position ] ) */           \
+  CPP(StringPrototypeStartsWith)                                              \
   /* ES6 section 21.1.3.25 String.prototype.toString () */                    \
   TFJ(StringPrototypeToString, 1)                                             \
   CPP(StringPrototypeTrim)                                                    \