When accessing JSArray::length property from GenericPropertyLoad
(i.e. via a megamorphic KEYED_LOAD_IC), we'd always go to the runtime
at this point, because the CallGetterIfAccessor method didn't support
AccessorInfos at all. Now there's initial support for JSArray::length,
which reduces the number of %KeyedGetProperty calls we see in the
Speedometer/EmberJS test by 5000.

Also-By: ishell@chromium.org
BUG=v8:5269
R=ishell@chromium.org

Change-Id: I44ce7966f9b7257808110a24d95a8167ab035df9
Reviewed-on: https://chromium-review.googlesource.com/488224Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44915}