This CL implements the functionality of SafeRemoveArrayHoles (JS),
which is used as a pre-processing step for sorting, in a runtime
function.

SafeRemoveArrayHoles is a generic fallback, when an existing runtime
function fails to remove holes/move undefineds to the end of an array.

This CL extends the existing runtime function to also support JSProxy
objects, and objects where indices have accessors.

R=cbruni@chromium.org, jgruber@chromium.org

Bug: v8:7382
Change-Id: I4881539cf2171caba08ff6e3e50320291f49839c
Reviewed-on: https://chromium-review.googlesource.com/1041950
Commit-Queue: Simon Zünd <szuend@google.com>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53060}