Previously we would shift the length of the string by three, which
could overflow with the new larger string length limit. Now we check
that the length will fit without extra allocation before and after
the shift, because really large strings will never fit, and will
always go to the Checked case.

Bug: chromium:748069, v8:6148
Change-Id: I41cac14b0fde6c5e8ca92305a052cbb743111554
Reviewed-on: https://chromium-review.googlesource.com/584611
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46896}