The spec was modified to relax some requirements which implementors had not been
enforcing. Part of this process involved introducing a new abstract operation
ToIndex, which had partial overlap with our existing semantics as well as some
differences (most notably treating undefined as 0). Test262 tests were introduced to
check for the new semantics, some of which we were failing. This patch amends the
parts of our implementation corresponding to specification algorithms which use
ToIndex to follow its semantics precisely.

BUG=v8:4784,v8:5120

Review-Url: https://codereview.chromium.org/2090353003
Cr-Commit-Position: refs/heads/master@{#37406}