Add a CheckSmi call to the value of the position argument to
String.prototype.startsWith(search, [position]).

Bug: v8:8400, chromium:939746
Change-Id: I7462bebe0d3fde605a4c27a34c0d9bb3f0cc1c20
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1514198
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60216}