- Remove the hack the resets the new space top pointer from SeqString::Truncate.
- Remove NewSpace::set_top completely as there are no callers (and there should
  be no!)

R=hpayer@chromium.org
BUG=chromium:536163
LOG=N

Review URL: https://codereview.chromium.org/1376753002

Cr-Commit-Position: refs/heads/master@{#31004}