If it points to a new space object which doesn't get scavenged, we need to drop
the reference.

BUG=v8:7308, v8:7768

Change-Id: I4485a7abcac3a26781811cc9bf134fd80e5f35b5
Reviewed-on: https://chromium-review.googlesource.com/1069127
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53287}