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This enables us to replace usage of mojo::SharedRemote and the thread
pool usage with an ordinary mojo::Remote. This should be a performance
improvement as it eliminates a hop through the thread pool.

Adds usage of sequence checker to assert thread safety expectations.

Change-Id: I93a681a2a6543fed417dcffbd0b43da121caddf5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2439559Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: Darin Fisher <darin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812414}