Fix reftest crash logs

Fix reftest crash logs

This patch fixes a long-standing bug where crashing reftests would not
show some crash logs on the layout tests results page.

single_test_runner.py has an optimization to not run reftests when
the base test crashes. This optimization accidentally failed to write
crash logs (done through write_test_result(...)) as well. This patch
fixes this bug and adds a small test.

This is a reland of https://codereview.chromium.org/1312693002 which
was rolled out due to crashes on Win10 (see http://crbug.com/524248#50).
This patch has two potential fixes for the crashes seen before:
1) test results are only written for crash tests, not timeouts.
2) The non-null expected_driver_output is used in the call for
write_test_result instead of reference_output which is null and
caused asserts.

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=201827

[pdr., 2015-09-04 05:05:33]

The CQ bit was checked by pdr@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-09-04 05:05:35]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1313533004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1313533004/1

[commit-bot: I haz the power, 2015-09-04 05:46:31]

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

[commit-bot: I haz the power, 2015-09-04 05:46:31]

Dry run: This issue passed the CQ dry run.

[pdr., 2015-09-04 17:31:05]

pdr@chromium.org changed reviewers:
+ dpranke@chromium.org, estaab@chromium.org

[pdr., 2015-09-04 17:32:15]

@dpranke, I'm the gardener today and would like to try relanding this. I've
updated the patch to only do what it says (write crash test logs) so it should
no longer crash on the bots.

[Dirk Pranke, 2015-09-04 22:21:15]

lgtm

[pdr., 2015-09-04 22:22:22]

The CQ bit was checked by pdr@chromium.org

[commit-bot: I haz the power, 2015-09-04 22:22:39]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1313533004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1313533004/1

[commit-bot: I haz the power, 2015-09-04 22:27:20]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201827