The fast paths aren't pure and have side effects like calling out to
the debugger and runtime calls. Note: These aren't "fast" paths per se,
but just *native promise* code paths.

Forcing the slow path omits these calls to the debugger and runtime
causing test failures.

Bug: v8:7148
Change-Id: Idf46a33622a6edf03d69fefa4c6bfb7efc8ea625
Reviewed-on: https://chromium-review.googlesource.com/824102Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50197}