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This change reworks the decision and hint logic to prepare for integrating
with the optimization guide. The observer functions with an asynchronous
callback so that hints can come either from the optimization guide or the
current hint mechanism via Finch.

The next change will start the integration with the optimization guide
behind a flag.

Bug: 1117064
Change-Id: I320635e795aafa2db2b2d413f766f94a4077cb9b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2367024Reviewed-by: Sophie Chang <sophiechang@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Michael Crouse <mcrouse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800377}