Fix a DCHECK violation in ipc_perftests

Fix a DCHECK violation in ipc_perftests

We only allow one MessageLoop instance alive at any given time on a thread. If
we use IPCTestBase::set_message_loop(), we need to do it in two steps: first set
the message loop to NULL, and then set it to a newly-created message loop.

Therefore, I changed the interface of IPCTestBase to make it a little
more friendly.

BUG=None

TEST=ipc_perftests should not crash in debug build.

Committed: https://crrev.com/22796d8d5fd362111b7aa431d229aa2ffc91aedd
Cr-Commit-Position: refs/heads/master@{#293439}

[yzshen1, 2014-09-03 18:00:40]

yzshen@chromium.org changed reviewers:
+ jam@chromium.org, morrita@chromium.org

[yzshen1, 2014-09-03 18:02:02]

Hi, Hajime and John.

Would you please take a look? Thanks!

Hajime: everything.
John: high-level idea and OWNER.

[Hajime Morrita, 2014-09-03 19:06:04]

lgtm. thanks for the fix.
Still needs lgtm from jam@

[jam, 2014-09-04 22:24:20]

lgtm

[yzshen1, 2014-09-04 22:24:31]

The CQ bit was checked by yzshen@chromium.org

[commit-bot: I haz the power, 2014-09-04 22:26:48]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/yzshen@chromium.org/538593002/20001

[commit-bot: I haz the power, 2014-09-05 04:43:39]

Committed patchset #2 (id:20001) as e61fbc2d16c4f5476b92e242332436a161f57ae6

[commit-bot: I haz the power, 2014-09-10 03:36:48]

Patchset 2 (id:??) landed as
https://crrev.com/22796d8d5fd362111b7aa431d229aa2ffc91aedd
Cr-Commit-Position: refs/heads/master@{#293439}