This fixes a bug introduced in crrev.com/c/2660995.
String.prototype.indexOf must clamp the `position` argument as per
step:

  7. Let start be the result of clamping pos between 0 and len.

Source: tc39.es/ecma262/#sec-string.prototype.indexof

Previously, this was done in the StringIndexOf builtin, but the recent
refactor changed builtin implementations to match the spec more
closely (i.e. to clamp in String.prototype.indexOf, not
StringIndexOf). This means we now have to clamp in
JSCallReducer::ReduceStringPrototypeIndexOf.

Tbr: neis@chromium.org
Bug: chromium:1194869
Change-Id: I5af8d41b50f4905453f03079e3ee6d46186536db
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2799359Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73772}