Previously (since f0e95769), this toLowerCase fast-path assumed
it would only see one-byte flat contents. Unfortunately, it's 
possible to have a one-byte sliced string that has a two-byte parent.

This CL ensures that String.p.toLowerCase handles such cases
correctly.

BUG=chromium:736451

Cq-Include-Trybots: master.tryserver.v8:v8_linux_noi18n_rel_ng
Change-Id: Iae056b3db5535bb5665439a5cc8282a51571a548
Reviewed-on: https://chromium-review.googlesource.com/565559Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46574}