Use ThreadChecker in HistogramDeltaSerialization class.

Use ThreadChecker in HistogramDeltaSerialization class.

There's a suspicion that this code might be being used from multiple threads. Add a ThreadChecker to make sure this isn't happening.

NOTE: These checks will still only be active in debug builds, but if they pass all the bots (that do run with dchecks on) and for developer builds, then I think it's pretty safe to assume there's no issue here. But good to have the precaution.

BUG=554011
Committed: https://crrev.com/0b6761ff0d1339e11707b6459152396320acc1b8
Cr-Commit-Position: refs/heads/master@{#360689}

[Alexei Svitkine (slow), 2015-11-19 20:14:13]

Patchset #1 (id:1) has been deleted

[Alexei Svitkine (slow), 2015-11-19 22:04:28]

Description was changed from

==========
.

BUG=
==========

to

==========
Use ThreadChecker in HistogramDeltaSerialization class.

There's a suspicion that this code might be being used from multiple threads.
Add a ThreadChecker to make sure this isn't happening.

NOTE: These checks will still only be active in debug builds, but if they pass
all the bots (that do run with dchecks on) and for developer builds, then I
think it's pretty safe to assume there's no issue here. But good to have the
precaution.

BUG=554011
==========

[Alexei Svitkine (slow), 2015-11-19 22:06:09]

asvitkine@chromium.org changed reviewers:
+ wfh@chromium.org

[Will Harris, 2015-11-19 22:35:36]

lgtm. will you consider changing these to CHECK() if no failures are seen on the
waterfall?

[Alexei Svitkine (slow), 2015-11-19 23:09:41]

On 2015/11/19 22:35:36, Will Harris wrote:
> lgtm. will you consider changing these to CHECK() if no failures are seen on
the
> waterfall?

Yeah, we can try that afterwards.

[Alexei Svitkine (slow), 2015-11-19 23:09:47]

The CQ bit was checked by asvitkine@chromium.org

[commit-bot: I haz the power, 2015-11-19 23:12:19]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1463013002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1463013002/20001

[commit-bot: I haz the power, 2015-11-19 23:49:56]

Committed patchset #1 (id:20001)

[commit-bot: I haz the power, 2015-11-19 23:50:54]

Patchset 1 (id:??) landed as
https://crrev.com/0b6761ff0d1339e11707b6459152396320acc1b8
Cr-Commit-Position: refs/heads/master@{#360689}