Visitor id for JSObjects does not depend on the instance size.

BUG=chromium:694255

Change-Id: I6b4b559207609f4a4c19373572be6692a55c5187
Reviewed-on: https://chromium-review.googlesource.com/520763Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45650}