(Simplifying the code in preparation for in-place weak references.)

BUG=v8:7308

Change-Id: I8348e108ae12bcd5b218f2d987f34017a1eb5394
Reviewed-on: https://chromium-review.googlesource.com/928421
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51424}