cc: Fix use-after-free in test.

cc: Fix use-after-free in test.

The following sequence of events causes a crash in this test:
1. LayerTreeTest::RealEndTest is called when the test ends.
2. LayerTreeTest::layer_tree_host_ is set to nullptr.
3. A task posted by the FakeExternalBeginFrameSource runs before
   ThreadProxy::LayerTreeHostClosedOnImplThread is called.
4. We try to dereference LayerTreeTest::layer_tree_host_ by calling
   LayerTreeTest::ImplThreadTaskRunner.

The fix is to simply not use LayerTreeTest::ImplThreadTaskRunner after
the test has ended.

BUG=476094
R=danakj
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Committed: https://crrev.com/5f4f6a4984527f5fca0be5972e7f884180c770d9
Cr-Commit-Position: refs/heads/master@{#346297}

[sunnyps, 2015-08-28 23:55:14]

PTAL

[sunnyps, 2015-08-28 23:55:14]

PTAL

[danakj, 2015-08-28 23:59:23]

https://codereview.chromium.org/1312123005/diff/1/cc/trees/layer_tree_host_un...
File cc/trees/layer_tree_host_unittest.cc (right):

https://codereview.chromium.org/1312123005/diff/1/cc/trees/layer_tree_host_un...
cc/trees/layer_tree_host_unittest.cc:2027:
base::ThreadTaskRunnerHandle::Get()->PostTask(
Could you just early out if TestEnded() ?

[sunnyps, 2015-08-29 00:08:26]

PTAL

https://codereview.chromium.org/1312123005/diff/1/cc/trees/layer_tree_host_un...
File cc/trees/layer_tree_host_unittest.cc (right):

https://codereview.chromium.org/1312123005/diff/1/cc/trees/layer_tree_host_un...
cc/trees/layer_tree_host_unittest.cc:2027:
base::ThreadTaskRunnerHandle::Get()->PostTask(
On 2015/08/28 23:59:23, danakj wrote:
> Could you just early out if TestEnded() ?

Done.

[danakj, 2015-08-29 00:15:05]

https://codereview.chromium.org/1312123005/diff/20001/cc/trees/layer_tree_hos...
File cc/trees/layer_tree_host_unittest.cc (right):

https://codereview.chromium.org/1312123005/diff/20001/cc/trees/layer_tree_hos...
cc/trees/layer_tree_host_unittest.cc:2027: if (TestEnded())
Cool! Can you just leave a comment here explaining why the early out? Then LGTM

[sunnyps, 2015-08-29 00:30:52]

Thanks!

https://codereview.chromium.org/1312123005/diff/20001/cc/trees/layer_tree_hos...
File cc/trees/layer_tree_host_unittest.cc (right):

https://codereview.chromium.org/1312123005/diff/20001/cc/trees/layer_tree_hos...
cc/trees/layer_tree_host_unittest.cc:2027: if (TestEnded())
On 2015/08/29 00:15:05, danakj wrote:
> Cool! Can you just leave a comment here explaining why the early out? Then
LGTM

Done.

[sunnyps, 2015-08-29 00:30:54]

The CQ bit was checked by sunnyps@chromium.org

[sunnyps, 2015-08-29 00:30:55]

The patchset sent to the CQ was uploaded after l-g-t-m from danakj@chromium.org
Link to the patchset: https://codereview.chromium.org/1312123005/#ps40001
(title: "comment")

[commit-bot: I haz the power, 2015-08-29 00:31:33]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1312123005/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1312123005/40001

[commit-bot: I haz the power, 2015-08-29 01:44:17]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-08-29 01:45:07]

Patchset 3 (id:??) landed as
https://crrev.com/5f4f6a4984527f5fca0be5972e7f884180c770d9
Cr-Commit-Position: refs/heads/master@{#346297}