Bug 1381876 - Introduce TimedPromise. r=automatedtester

This introduces a specialisation of the well-known Promise that can
time out after a set limit, causing the promises' "reject" callback to
be invoked.

The TimedPromise object represents the timed, eventual completion
(or failure) of an asynchronous operation, and its resulting value.
In contrast to a regular Promise, it times out after a set timeframe.

MozReview-Commit-ID: Rb3POsPYeT

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extra : rebase_source : cdff792dfa43af3cff1b20b3e81edb1a826e561e