Previously, for the various customisation points of String builtins
(like String.prototype.replace), we skipped the customisation symbol
lookup (like for Symbol.replace) for Smis.

But, we do need to do the lookup for Smis in case Number.prototype or
Object.prototype have the Symbol. This missing lookup was creating an
observable difference between Smis and HeapNumbers.

Bug: chromium:1092896
Change-Id: I8928d237fa74abeaa2aa81318b8903087c507f0d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2238030
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68285}