[heap] Remove NewSpace::set_top and the corresponding hack

[heap] Remove NewSpace::set_top and the corresponding hack

- 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

Committed: https://crrev.com/92a7012b8b22227167eb1212c9d1592cf2e37510
Cr-Commit-Position: refs/heads/master@{#31004}

[Michael Lippautz, 2015-09-29 12:40:27]

The CQ bit was checked by mlippautz@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-09-29 12:40:31]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1376753002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1376753002/1

[commit-bot: I haz the power, 2015-09-29 13:03:56]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-09-29 13:03:57]

Dry run: This issue passed the CQ dry run.

[Michael Lippautz, 2015-09-29 13:04:33]

The CQ bit was checked by mlippautz@chromium.org

[Michael Lippautz, 2015-09-29 13:04:39]

The CQ bit was unchecked by mlippautz@chromium.org

[Michael Lippautz, 2015-09-29 13:05:10]

mlippautz@chromium.org changed reviewers:
+ hpayer@chromium.org

[Michael Lippautz, 2015-09-29 13:05:11]

Ali: FYI

[Hannes Payer (out of office), 2015-09-29 13:05:25]

lgtm

[Michael Lippautz, 2015-09-29 13:05:46]

The CQ bit was checked by mlippautz@chromium.org

[commit-bot: I haz the power, 2015-09-29 13:05:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1376753002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1376753002/1

[commit-bot: I haz the power, 2015-09-29 13:07:32]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-29 13:07:50]

Patchset 1 (id:??) landed as
https://crrev.com/92a7012b8b22227167eb1212c9d1592cf2e37510
Cr-Commit-Position: refs/heads/master@{#31004}