Write dm.json periodically instead of only once at the end.

Write dm.json periodically instead of only once at the end.

This way if a bot crashes, we might get some partial results in gold rather
than none.  We do the same sort of thing in nanobench for perf.

BUG=skia:3255
Committed: https://skia.googlesource.com/skia/+/a17241bd0a59efd6c30a007db8f10274b8f80f26

[mtklein_C, 2015-01-22 21:51:55]

The CQ bit was checked by mtklein@chromium.org

[mtklein_C, 2015-01-22 21:52:08]

mtklein@chromium.org changed reviewers:
+ jcgregorio@google.com, stephana@google.com

[commit-bot: I haz the power, 2015-01-22 21:52:51]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/872443003/1

[commit-bot: I haz the power, 2015-01-22 21:52:52]

Note for Reviewers:
The CQ is waiting for an approval.
If you believe that the CL is not ready yet, or if you would like to L-G-T-M
with comments then please uncheck the CQ checkbox.

Waiting for LGTM from valid reviewer(s) till 2015-01-23 03:52 UTC

[commit-bot: I haz the power, 2015-01-23 00:54:32]

Note for Reviewers:
The CQ is waiting for an approval.
If you believe that the CL is not ready yet, or if you would like to L-G-T-M
with comments then please uncheck the CQ checkbox.

Waiting for LGTM from valid reviewer(s) till 2015-01-23 06:54 UTC

[commit-bot: I haz the power, 2015-01-23 06:54:38]

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

[commit-bot: I haz the power, 2015-01-23 06:54:39]

No LGTM from a valid reviewer yet. Please ask for an LGTM from a
full Skia committer from
https://skia.googlesource.com/skia/+/master/CQ_COMMITTERS

[stephana, 2015-01-23 13:25:54]

On 2015/01/23 06:54:39, I haz the power (commit-bot) wrote:
> No LGTM from a valid reviewer yet. Please ask for an LGTM from a
> full Skia committer from
> https://skia.googlesource.com/skia/+/master/CQ_COMMITTERS

LGTM

[mtklein, 2015-01-23 13:46:49]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-01-23 13:47:40]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/872443003/1

[commit-bot: I haz the power, 2015-01-23 13:48:04]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/a17241bd0a59efd6c30a007db8f10274b8f80f26