This CL changes the builtin
  StringSubstring(string, start, end)
to take start and end as untagged IntPtr values.

Bug: v8:7250, v8:7340

Change-Id: I39700d087da903f076a6ca163a8f880d31eea3a0
Reviewed-on: https://chromium-review.googlesource.com/923961
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51556}