Now that we always instantiate the right ObjectData subclass, we can
give precise types to members.

R=jarin@chromium.org

Bug: v8:7790
Change-Id: Ic2194de90f458ddccbeb9f101903e5865fb4eb41
Reviewed-on: https://chromium-review.googlesource.com/1187103Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55404}