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- Use a ThreadChecker member instead of inheriting from ThreadChecker.
- Switch from a leaky Singleton to base::NoDestructor.

Change-Id: I86f8f50f7776ceefa62ab3586314c5af3bacf7e7
Reviewed-on: https://chromium-review.googlesource.com/1178468Reviewed-by: Jenny Zhang <jennyz@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583866}