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Collects stack samples at 10 Hz for 30 seconds at startup. Uploading of
samples to UMA is controlled by a Finch experiment.

This change moves to a callback-based mechanism for communicating
completed profiles to the metrics provider to avoid profiles lingering
in memory when reporting is disabled.

BUG=464929

Review URL: https://codereview.chromium.org/1029653002

Cr-Commit-Position: refs/heads/master@{#324175}