The fast-path of S.p.charAt/charCodeAt wrongly truncates the index,
i.e. charAt(k + 4294967295) yields the same as charAt(k-1). This CL
fixes this behaviour, at the cost of not providing a fast-path for
charAt(1.1), i.e. if charAt/charCodeAt is called with a Number.

Bug: chromium:800594
Change-Id: Ic8e749380d3118f0c9469eb626e81bf72cf09fec
Reviewed-on: https://chromium-review.googlesource.com/860003Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50497}