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This initially introduces a subset of PreviewsHints logic into this new
service to incorporate the component hints and puts it into the store
without any mechanisms for retrieving it out of the store (that will
follow in subsequent CLs). Following this new path is subject to the new
"OptimizationGuideKeyedService" Feature being enabled.

Once this new path for loading and managing hints is fully landed, the logic
will be removed from PreviewsHints and PreviewsOptimizationGuide.

Bug: 969558
Change-Id: Ieebaf137e1b93dadc55dc381d3edad0606fe7b16
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1709652
Commit-Queue: Sophie Chang <sophiechang@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Doug Arnett <dougarnett@chromium.org>
Reviewed-by: Stefan Kuhne <skuhne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681450}