In the process, create a shared array utility GetLengthProperty that fast-paths
accessing the length properties of JSArray.

Bug: v8:7793
Change-Id: I6d7f0007c162794773dc0fc3e8bf12b3adf12fa0
Reviewed-on: https://chromium-review.googlesource.com/1116221
Commit-Queue: Daniel Clifford <danno@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54133}