base: Use nullptr instead of NULL and static_cast in discardable memory unit tests.

base: Use nullptr instead of NULL and static_cast in discardable memory unit tests.

BUG=422953
TEST=base_unittests --gtest_filter=DiscardableMemoryTests*
TBR=danakj@chromium.org

Committed: https://crrev.com/d8ee1714ece7a563f981813381610fd37c37444e
Cr-Commit-Position: refs/heads/master@{#299991}

[reveman, 2014-10-16 20:46:54]

reveman@chromium.org changed reviewers:
+ avi@chromium.org

[reveman, 2014-10-16 20:47:04]

The CQ bit was checked by reveman@chromium.org

[commit-bot: I haz the power, 2014-10-16 20:48:07]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/659143002/1

[danakj, 2014-10-16 20:51:05]

LGTM

[commit-bot: I haz the power, 2014-10-16 22:20:29]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-16 22:21:20]

Patchset 1 (id:??) landed as
https://crrev.com/d8ee1714ece7a563f981813381610fd37c37444e
Cr-Commit-Position: refs/heads/master@{#299991}

[Avi (use Gerrit), 2014-10-17 19:41:00]

lgtm