The start argument must be converted to an integer before the length argument is
converted.  (Consequently, the start argument is converted even when the length
is 0.)  This matters because conversion is observable.

Also rewrite the function in a way that closely resembles the spec text.

R=littledan@chromium.org
BUG=v8:5140

Review-Url: https://codereview.chromium.org/2109583002
Cr-Commit-Position: refs/heads/master@{#37378}