[Mac] Guard against null memory pressure dispatch source

[Mac] Guard against null memory pressure dispatch source

BUG=673822
Committed: https://crrev.com/d3eb66287d102d4fa4f8750e39ec4d1eb97c3b71
Cr-Commit-Position: refs/heads/master@{#438317}

[lgrey, 2016-12-13 20:57:15]

lgrey@chromium.org changed reviewers:
+ rsesek@chromium.org

[lgrey, 2016-12-13 20:57:15]

This is a speculative fix for the attached bug. I can't repro on a 10.9.5
machine, and the 10.8 -> 10.9 API diffs confirm that
DISPATCH_SOURCE_TYPE_MEMORYPRESSURE was introduced in 10.9. 

The one thing I can think of is that it previously expanded to something besides
_dispatch_source_type_memorypressure. I can't find a way to link public
libDispatch source versions to the OS version they shipped with, so can't
confirm.

[Robert Sesek, 2016-12-13 21:03:02]

Can the reporter repro this on current canary? If so, then we can land this and
have them test again to see if this fixes it.

[Robert Sesek, 2016-12-13 21:16:35]

LGTM based on https://crbug.com/673822#c8 though.

[lgrey, 2016-12-13 21:22:08]

lgrey@chromium.org changed reviewers:
+ mark@chromium.org

[lgrey, 2016-12-13 21:22:08]

+mark@ for base OWNERS

[Mark Mentovai, 2016-12-13 21:43:55]

LGTM

[lgrey, 2016-12-13 21:55:25]

The CQ bit was checked by lgrey@chromium.org

[commit-bot: I haz the power, 2016-12-13 21:56:04]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-12-13 22:41:54]

CQ is committing da patch.

Bot data: {"patchset_id": 1, "attempt_start_ts": 1481666125473110, "parent_rev":
"e8239a2f93c44d2d21af21b431942b7cb3670c82", "commit_rev":
"25e77941e39bffb823c1fdf2f0293da33407c99b"}

[commit-bot: I haz the power, 2016-12-13 22:42:25]

Description was changed from

==========
[Mac] Guard against null memory pressure dispatch source

BUG=673822
==========

to

==========
[Mac] Guard against null memory pressure dispatch source

BUG=673822

Review-Url: https://codereview.chromium.org/2570623004
==========

[commit-bot: I haz the power, 2016-12-13 22:42:26]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2016-12-13 22:43:57]

Description was changed from

==========
[Mac] Guard against null memory pressure dispatch source

BUG=673822

Review-Url: https://codereview.chromium.org/2570623004
==========

to

==========
[Mac] Guard against null memory pressure dispatch source

BUG=673822

Committed: https://crrev.com/d3eb66287d102d4fa4f8750e39ec4d1eb97c3b71
Cr-Commit-Position: refs/heads/master@{#438317}
==========

[commit-bot: I haz the power, 2016-12-13 22:43:58]

Patchset 1 (id:??) landed as
https://crrev.com/d3eb66287d102d4fa4f8750e39ec4d1eb97c3b71
Cr-Commit-Position: refs/heads/master@{#438317}