The goal is to remove CL to remove the confusing implications for
full poisoning.

This is an alternative to
https://chromium-review.googlesource.com/c/chromium/src/+/1253341
where chrome has to work around our implication system.

In the optimizing compiler, we already have a bottleneck for setting
mitigation level in src/compiler/pipeline.cc, so it is easy to change
back to partial mitigations.

Bug: chromium:888892
Change-Id: I01de7ed7bb91e8b06f8f79cc2d90657a0600892a
Reviewed-on: https://chromium-review.googlesource.com/c/1252985Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56374}