base/task/: public APIs for posting tasks and managing task queues.
base/task/thread_pool/: implementation of the ThreadPool.
base/task/sequence_manager/: implementation of the SequenceManager.
base/task/common/: implementation details shared by ThreadPool and
SequenceManager.
Apart from embedders explicitly managing a ThreadPoolInstance and/or
SequenceManager instance(s) for their process/threads, the vast majority of
users should only need APIs in base/task/.