There's no reason to use our self-baked atomics anymore. Also

- Changes two boolean values to use a boolean instead of an int
- Uses a unique ptr for data_
- Removes has_processing_thread_ which is not used
- Moves most initialization inline into the class
- Removes SetUp/TearDown which weren't needed

Change-Id: I8f50133636961502d56351abd2fb17196603a01a
Reviewed-on: https://chromium-review.googlesource.com/c/1422918
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58950}