This includes a fastpath in the ElementsAccessor for the source
array being a JSArray with FastSmi or FastDouble packed kinds. This
is probably a pretty common usage, where an array is passed in as
a way of initializing the TypedArray at creation (as there is not other
syntax to do this). e.g. new Float64Array([1.0, 1.0, 1.0]) for some
sort of vector application.

BUG= v8:5977

Change-Id: Ice4ad9fc29f56b1c4b0b30736a1330efdc289003
Reviewed-on: https://chromium-review.googlesource.com/465126Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44722}