Bug 1040593 - Partly allocate elements for |new Array(N)| when N > 2048. r=jandem,bhackett.

This patch makes |new Array(N)| always allocate at least
|EagerAllocationsMaxLength| elements up front.

It also changes |EagerAllocationsMaxLength| from 2048 to 2046, so that when the
two extra slots needed for |ObjectElements| are included, the resulting
allocation size is a power-of-two.

--HG--
extra : rebase_source : 17e069bc9d39c5d5ee957a99e11ab189f7493540