scheduler: Don't run multiple tasks per batch in a nested run loop

scheduler: Don't run multiple tasks per batch in a nested run loop

If the scheduler is in a nested run loop, it mustn't run multiple tasks
per batch. This is because one of those tasks could be a call to
RunLoop::Quit(), which means we have to yield control back to the
message loop. Since we have no direct way of observing when Quit is
called, just limit the number of tasks per batch to 1 in nested run
loops.

BUG=444764
Committed: https://crrev.com/418ebef4953dba6f31d7b16d9b4d1565c8b22a83
Cr-Commit-Position: refs/heads/master@{#340133}

[Sami, 2015-07-23 16:37:45]

skyostil@chromium.org changed reviewers:
+ alexclarke@chromium.org

[alex clarke (OOO till 29th), 2015-07-23 16:41:09]

lgtm

[Sami, 2015-07-23 16:52:16]

The CQ bit was checked by skyostil@chromium.org

[Sami, 2015-07-23 16:52:17]

The patchset sent to the CQ was uploaded after l-g-t-m from
alexclarke@chromium.org
Link to the patchset: https://codereview.chromium.org/1252143002/#ps20001
(title: "Rebased.")

[commit-bot: I haz the power, 2015-07-23 16:52:46]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1252143002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1252143002/20001

[commit-bot: I haz the power, 2015-07-23 17:04:44]

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

[commit-bot: I haz the power, 2015-07-23 17:04:45]

Try jobs failed on following builders:
  linux_chromium_gn_dbg on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[Sami, 2015-07-23 17:16:19]

The CQ bit was checked by skyostil@chromium.org

[Sami, 2015-07-23 17:16:19]

The patchset sent to the CQ was uploaded after l-g-t-m from
alexclarke@chromium.org
Link to the patchset: https://codereview.chromium.org/1252143002/#ps40001
(title: "Build fix.")

[commit-bot: I haz the power, 2015-07-23 17:16:58]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1252143002/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1252143002/40001

[commit-bot: I haz the power, 2015-07-23 17:43:37]

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

[commit-bot: I haz the power, 2015-07-23 17:43:38]

Try jobs failed on following builders:
  linux_chromium_chromeos_rel_ng on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[Sami, 2015-07-23 17:52:47]

The CQ bit was checked by skyostil@chromium.org

[commit-bot: I haz the power, 2015-07-23 17:53:07]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1252143002/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1252143002/40001

[commit-bot: I haz the power, 2015-07-23 18:05:32]

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

[commit-bot: I haz the power, 2015-07-23 18:05:33]

Try jobs failed on following builders:
  linux_chromium_asan_rel_ng on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[Sami, 2015-07-23 18:09:08]

The CQ bit was checked by skyostil@chromium.org

[commit-bot: I haz the power, 2015-07-23 18:09:28]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1252143002/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1252143002/40001

[commit-bot: I haz the power, 2015-07-23 19:08:47]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-07-23 19:10:04]

Patchset 3 (id:??) landed as
https://crrev.com/418ebef4953dba6f31d7b16d9b4d1565c8b22a83
Cr-Commit-Position: refs/heads/master@{#340133}