This fixes String::MakeExternal() to bail out if the subject string
doesn't fit a regular ExternalString, instead of creating a short
external string. The observation here is that for short external strings
the overhead of having to have the StringResource plus going to the
runtime/C++ for each and every character access from JavaScript land
is probably bigger than the anticipated benefits.

If this turns out to be wrong and there's a real benefit, we should make
use of ThinStrings instead of having a separate way to represent
external strings.

Bug: v8:6621, v8:7109, v8:7145
Change-Id: I4b75da08b82a72027c782a69de9c8eaf3cca1d4d
Reviewed-on: https://chromium-review.googlesource.com/799750Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49735}