Move invalid string length check to the factory.

src/factory.h

Index: src/factory.h
diff --git a/src/factory.h b/src/factory.h
index 7ef8911d702e4add0260ca2d4c2073e4ac354d5a..e22ea8d323d4aa45ff1f1bd8bbe2b1d268511c69 100644
--- a/src/factory.h
+++ b/src/factory.h
@@ -122,6 +122,23 @@ class Factory V8_FINAL {
         OneByteVector(str), pretenure).ToHandleChecked();
   }
 
+
+  // Allocates and fully initializes a String.  There are two String
+  // encodings: ASCII and two byte. One should choose between the three string
+  // allocation functions based on the encoding of the string buffer used to
+  // initialized the string.
+  //   - ...FromAscii initializes the string from a buffer that is ASCII
+  //     encoded (it does not check that the buffer is ASCII encoded) and the
+  //     result will be ASCII encoded.
+  //   - ...FromUTF8 initializes the string from a buffer that is UTF-8
+  //     encoded.  If the characters are all single-byte characters, the
+  //     result will be ASCII encoded, otherwise it will converted to two
+  //     byte.
+  //   - ...FromTwoByte initializes the string from a buffer that is two-byte
+  //     encoded.  If the characters are all single-byte characters, the
+  //     result will be converted to ASCII, otherwise it will be left as
+  //     two-byte.
+
   // TODO(dcarney): remove this function.
   MUST_USE_RESULT inline MaybeHandle<String> NewStringFromAscii(
       Vector<const char> str,