This CL moves Array.p.fill from JavaScript to a C++ builtin. It has
a generic slow-path and fast-paths implemented via ElementsAccessor in
elements.cc.

R=cbruni@chromium.org

Bug: v8:7624
Change-Id: I8820e1195d2cd9b41c254058923ad9875aab067c
Reviewed-on: https://chromium-review.googlesource.com/1131130
Commit-Queue: Simon Zünd <szuend@google.com>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54522}