Migrate feature_engagement::Tracker::OnInitializedCallback to OnceCallback.

Bug: 714018
Change-Id: I2085e52ccd2e6f9205a0dc7b10bd9dd7c519ae84
Reviewed-on: https://chromium-review.googlesource.com/1117468
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Commit-Queue: Sky Malice <skym@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571645}
4 files changed