It is possible for user code to modify fast regexp result objects
before they are used e.g. by RegExp.p.match, so we may not make any
assumptions about their contents. The only exception is when the
RegExp itself is fast.

Bug: chromium:843022
Change-Id: I14eafbdfb2b2ced609da1391b57c73cbe167f7fb
Reviewed-on: https://chromium-review.googlesource.com/1061455Reviewed-by: Peter Wong <peter.wm.wong@gmail.com>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53210}