LayoutDescriptor should inherit from JSTypedArray

It can't just inherit from a FixedTypedArray-like type, as we soon
assume that a FixedTypedArray-like type is always held by an
ArrayBufferView-like type

BUG=v8:3996
R=ishell@chromium.org,verwaest@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1084793004

Cr-Commit-Position: refs/heads/master@{#27964}