Issue 6051 description: "Blink blindly assumes that it can get an
aligned pointer from the 0-th internal field of any object that has two
internal fields."

R=titzer@chromium.org, jochen@chromium.org

BUG=v8:6051

Change-Id: I814b76e508ffd9fe2326bd0e728129f2a013b807
Reviewed-on: https://chromium-review.googlesource.com/451319Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43653}