MakeNotNull is similar to UniquePtr, in that it combines the infallible
allocation and construction of an object on the heap and wraps the (raw or
smart) pointer into a NotNull.
It skips the unnecessary null check from WrapNotNull, and removes the usual
naked 'new' used in many WrapNotNull calls.
MozReview-Commit-ID: UwCrhDnkUg
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extra : rebase_source : 5a027165fc17ed748783c7ffda03eb421865ad6e
