Change ListContainer::AppendByMoving to return a pointer to the new item

Change ListContainer::AppendByMoving to return a pointer to the new item

This patch changes the return value of ListContainer::AppendByMoving to
return a pointer to the moved item at the new location. This was found
to be a performacne win in https://codereview.chromium.org/1193433004
because accessing the last element is not free. A new test has been
added to prove correctness.

BUG=484943
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Committed: https://crrev.com/3e92831524899eb6b6b3f7f61dd0f51dbd0ae7d6
Cr-Commit-Position: refs/heads/master@{#336647}

[pdr., 2015-06-29 20:19:09]

pdr@chromium.org changed reviewers:
+ chrishtr@chromium.org, danakj@chromium.org

[pdr., 2015-06-29 20:19:31]

pdr@chromium.org changed reviewers:
+ enne@chromium.org
- chrishtr@chromium.org

[pdr., 2015-06-29 20:19:40]

pdr@chromium.org changed reviewers:
- enne@chromium.org

[danakj, 2015-06-29 20:21:53]

LGTM

[pdr., 2015-06-29 20:22:45]

The CQ bit was checked by pdr@chromium.org

[commit-bot: I haz the power, 2015-06-29 20:24:47]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1219753003/1

[commit-bot: I haz the power, 2015-06-29 21:28:31]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-29 21:29:36]

Patchset 1 (id:??) landed as
https://crrev.com/3e92831524899eb6b6b3f7f61dd0f51dbd0ae7d6
Cr-Commit-Position: refs/heads/master@{#336647}