In the process, also enable support for PACKED_DOUBLE_ELEMENTS arrays.

Change-Id: I16dd79276f1023e30b072d45216396533077f53c
Reviewed-on: https://chromium-review.googlesource.com/571006
Commit-Queue: Daniel Clifford <danno@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48289}