Fast-to-slow migration should wipe out in-object space if it exists in the object after migration.

BUG=chromium:436816
LOG=N

Review URL: https://codereview.chromium.org/765473004

Cr-Commit-Position: refs/heads/master@{#25535}
Ship harmony-strings

R=rossberg@chromium.org, yangguo@chromium.org
LOG=Y

Review URL: https://codereview.chromium.org/761913002

Cr-Commit-Position: refs/heads/master@{#25536}