The assumption behind this optimization was invalid. Even if the
string's prototype is unchanged, the symbol could exist somewhere
further up the prototype chain.

GetProperty has been sped up significantly so it might be fine to just
skip this fast path. An alternative would be to use a protector cell.

Bug: v8:8357
Change-Id: Ia577107a58157350eb15780c02aa63d77e600637
Reviewed-on: https://chromium-review.googlesource.com/c/1301498
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57038}