This prefixes the flag to be more uniform with existing flags and makes
it available as a gn argument.

The switch feature is enabled on a subset of bots, the linux64 release
trybot and all sanitizers. Note, this doesn't switch to slow paths -
it just allows to use the runtime flag on those bots.

Bug: v8:7120
Change-Id: I706a7a9bd503c13d5432cac9a13a909f4f32a0a7
Reviewed-on: https://chromium-review.googlesource.com/798412Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49739}