Populate runtime experiment params and allow runtime feature enablement

`VariationsService` now copies the params of runtime-mutable experiments
from the seed into their `RuntimeFieldTrialInfo`, so
`FeatureParam<T>::Get()` returns them for the features that the
experiment enables. Experiments with params are no longer rejected, so
the `kRuntimeExperimentHasParams` histogram enum value is deprecated.

Runtime-mutable experiments that enable features can now be applied too,
as runtime mutability now supports both enabling and disabling features.
The `kNotStrictKillswitch` histogram enum value is deprecated.

The post-apply sanity check now verifies that each feature points to
the newly applied `RuntimeFieldTrialInfo` itself, rather than to one
with the same trial name, since that object now also carries the
params.

Bug: 536852160, 536851701
Change-Id: Ie44f65e067e61fb8133ff62c300a570bbda89544
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8419947
Commit-Queue: Roger McFarlane <rogerm@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Luc Nguyen <lucnguyen@google.com>
Cr-Commit-Position: refs/heads/main@{#1712284}