Introduce new runtime flag that forces to always use
slow path for regex, promise and array builtins. It
works in DEBUG or with new compile time flag
ENABLE_FASTSLOW_SWITCH.

It will be used in the fast/slow path fuzzer or as a
testing variant to ensure that slow path implementation
behave equivalent to corresponding fast paths (where
applicable).

Bug: v8:7120
Change-Id: Ia2a4ab7aca5051e852723782c529bd2e8e5925ca
Reviewed-on: https://chromium-review.googlesource.com/787291
Commit-Queue: Michał Majewski <majeski@google.com>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49642}