Allow for counterfactual logging the predictions coming from opt guide

This adds a feature parameter within the
LoadingPredictorUseOptimizationGuide feature to dictate whether to
actually use the predictions for preconnect or anything else later.

I tested via browser tests by removing the DISABLED_ locally. I'm not
remove the DISABLED_ in real code since I still haven't been able to
repro the flake locally and don't know the root cause of the flake on
bots.

Bug: 1064121
Change-Id: I6a47645662b04760520c8eff4dd52ec7f55ca064
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2119372
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Sophie Chang <sophiechang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754111}