ExternalString::kShortSize is not the same as i::kShortSize, caused
incorrect reporting for code stats for two byte strings.

Bug: chromium:837659
Change-Id: Icbb39f2103aa4fa72bd5b1258cb8e1d4aee10441
Reviewed-on: https://chromium-review.googlesource.com/1044212Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52992}