mojo: Fix Blink's shutdown sequence in AxProviderImplTest

mojo: Fix Blink's shutdown sequence in AxProviderImplTest

This patch ensures the sequence used to tear down Blink in
AxProviderImplTest matches the real implementation in RenderThreadImpl.
Specifically, we need to shut down the scheduler before terminating
Blink, because otherwise the scheduler might keep pointers to tasks
that live on the Blink heap and access freed memory in its destructor.

We also add an assertion to the renderer scheduler that it is shut down
explicitly.

Note that Blink cannot shut down the scheduler internally because of
crbug.com/467369. We expect that this depedency can be cleaned up once
the message loop is better integrated with the scheduler
(crbug.com/465354) and when the Blink repository has been merged into
Chromium.

BUG=463143
Committed: https://crrev.com/23e04e72e8121a372fcbaf779fde9faae2bc772f
Cr-Commit-Position: refs/heads/master@{#332221}

[Sami, 2015-06-01 11:24:17]

skyostil@chromium.org changed reviewers:
+ alexclarke@chromium.org, rmcilroy@chromium.org, sky@chromium.org

[alex clarke (OOO till 29th), 2015-06-01 11:38:54]

LGTM for scheduler

[rmcilroy, 2015-06-01 11:51:58]

LGTM for scheduler

[sky, 2015-06-01 15:17:21]

LGTM

[Sami, 2015-06-01 15:20:21]

The CQ bit was checked by skyostil@chromium.org

[commit-bot: I haz the power, 2015-06-01 15:21:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1145973011/1

[commit-bot: I haz the power, 2015-06-01 16:21:40]

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

[commit-bot: I haz the power, 2015-06-01 16:21:41]

Try jobs failed on following builders:
  linux_chromium_gn_rel on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[Sami, 2015-06-01 16:40:36]

The CQ bit was checked by skyostil@chromium.org

[Sami, 2015-06-01 16:40:36]

The patchset sent to the CQ was uploaded after l-g-t-m from sky@chromium.org,
rmcilroy@chromium.org, alexclarke@chromium.org
Link to the patchset: https://codereview.chromium.org/1145973011/#ps20001
(title: "Add missing shutdown call to html_viewer::Setup")

[commit-bot: I haz the power, 2015-06-01 16:41:15]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1145973011/20001

[commit-bot: I haz the power, 2015-06-01 18:16:56]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-06-01 18:18:49]

Patchset 2 (id:??) landed as
https://crrev.com/23e04e72e8121a372fcbaf779fde9faae2bc772f
Cr-Commit-Position: refs/heads/master@{#332221}