Double element types were much slower than Smi/Object previously.
We can box each double in a HeapNumber and push them into a new
FixedArray to save going into the runtime.

Bug: v8:4826, chromium:704966
Change-Id: I7f15d0d636a52760daefed722265c696c1ebb13e
Reviewed-on: https://chromium-review.googlesource.com/531004
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45897}