Array.indexOf accepts an optional fromIndex argument. When non-negative,
this argument restricts the searched indices to those starting at
fromIndex:
[1, 2, 1].indexOf(1,1) == 2
When negative, it is meant to be added to the array length to provide
such initial index for the search:
[1, 2, 1].indexOf(1, -2) == 2

This transformation has been done by the non-optimised builtin but not
by the reducer. The CL adds this construction to the reducer.

Bug: chromium:842612
Change-Id: I0ff089997f4ebb4dc3c2923e52c382a8a96cd711
Reviewed-on: https://chromium-review.googlesource.com/1059628Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Vaclav Brozek <vabr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53197}