Stop warnings resulting from unimplemented SetProcessBackgrounded().

Stop warnings resulting from unimplemented SetProcessBackgrounded().

Process::SetProcessBackgrounded() is not implemented on certain
platforms (e.g. OS X). Calling Process::SetProcessBackgrounded() on
these systems results in a warning to the console. Callers should
first check result of Process::CanBackgroundProcesses() before calling
SetProcessBackgrounded().

This cl also fixes a small error in the documentation for
SetProcessBackgrounded().

BUG=537301
Committed: https://crrev.com/7714401d923571957b492731a107bf0cd761fdde
Cr-Commit-Position: refs/heads/master@{#351943}

[shrike, 2015-09-29 21:52:03]

shrike@chromium.org changed reviewers:
+ jam@chromium.org, thestig@chromium.org

[shrike, 2015-09-29 21:52:28]

jam for content, thestig for base.

PTAL

[Lei Zhang, 2015-09-29 21:55:36]

lgtm

[shrike, 2015-09-30 22:09:40]

jam - are you available to take a look at this cl?

[shrike, 2015-10-01 22:43:39]

shrike@chromium.org changed reviewers:
+ avi@chromium.org
- jam@chromium.org

[shrike, 2015-10-01 22:44:08]

avi, PTAL (approval for content)

[Avi (use Gerrit), 2015-10-02 01:04:37]

lgtm

[Lei Zhang, 2015-10-02 01:10:15]

The CQ bit was checked by thestig@chromium.org

[commit-bot: I haz the power, 2015-10-02 01:10:29]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1378753002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1378753002/1

[commit-bot: I haz the power, 2015-10-02 01:20:00]

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

[commit-bot: I haz the power, 2015-10-02 01:20:01]

Try jobs failed on following builders:
  win_chromium_rel_ng on tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_rel_...)

[Lei Zhang, 2015-10-02 01:30:38]

The CQ bit was checked by thestig@chromium.org

[commit-bot: I haz the power, 2015-10-02 01:31:35]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1378753002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1378753002/1

[commit-bot: I haz the power, 2015-10-02 02:47:10]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-02 02:47:53]

Patchset 1 (id:??) landed as
https://crrev.com/7714401d923571957b492731a107bf0cd761fdde
Cr-Commit-Position: refs/heads/master@{#351943}