Fixes tray_session_length_limit_unittest timeout on valgrind.

Fixes tray_session_length_limit_unittest timeout on valgrind.

According to test runs on my machine, the test looks like suspended
after the test body finished, which seems to be waiting for
remaining timer tests. ClearSessionLengthLimit() should be called
at the end.

This also has cosmetic changes like NULL -> nullptr

BUG=454601
R=stevenjb@chromium.org
TEST=build with build_for_tool=memcheck, run ash_unittests with
  valgrind, and confirmed the test finishes in realistic time

Committed: https://crrev.com/75be556f6fc83ffdac88fd6ec12f846018aa8d2b
Cr-Commit-Position: refs/heads/master@{#314386}

[stevenjb, 2015-02-03 19:02:26]

lgtm

[Jun Mukai, 2015-02-03 19:04:38]

The CQ bit was checked by mukai@chromium.org

[commit-bot: I haz the power, 2015-02-03 19:05:57]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/895863002/1

[commit-bot: I haz the power, 2015-02-03 19:42:55]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-03 19:44:21]

Patchset 1 (id:??) landed as
https://crrev.com/75be556f6fc83ffdac88fd6ec12f846018aa8d2b
Cr-Commit-Position: refs/heads/master@{#314386}