Without this change, we could disable slow paths required when symbols
such as toStringTag are present on a receiver, but accessors or
interceptors are not (added in 31800120)

This change modifies this behaviour to not unset the previously set bit
if these forced slow path conditions are not met.

BUG=v8:7706
R=bmeurer@chromium.org

Change-Id: Id7bceb0e749da52e2dbcde0a310a865a89f24066
Reviewed-on: https://chromium-review.googlesource.com/1034210Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Cr-Commit-Position: refs/heads/master@{#52874}