scheduler: Remove task observer shenanigans

scheduler: Remove task observer shenanigans

Now that tasks posted to the message loop are routed through the
scheduler, there's no need for the scheduler to listen for task observer
notifications from the message loop or coalesce them into a mostly
consistent stream of notifications to downstream observers.

BUG=444764
Committed: https://crrev.com/e1ab6c84cbadc16f4d5d775e1ab3680fe0af2612
Cr-Commit-Position: refs/heads/master@{#339654}

[Sami, 2015-07-21 10:31:38]

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

[alex clarke (OOO till 29th), 2015-07-21 10:43:40]

lgtm

[Sami, 2015-07-21 10:46:38]

The CQ bit was checked by skyostil@chromium.org

[commit-bot: I haz the power, 2015-07-21 10:46:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1250613002/1

[commit-bot: I haz the power, 2015-07-21 11:18:54]

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

[commit-bot: I haz the power, 2015-07-21 11:18:55]

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

[Sami, 2015-07-21 14:26:23]

The CQ bit was checked by skyostil@chromium.org

[Sami, 2015-07-21 14:26:23]

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/1250613002/#ps20001
(title: "Fixed a failing test.")

[commit-bot: I haz the power, 2015-07-21 14:26:47]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1250613002/20001

[commit-bot: I haz the power, 2015-07-21 15:27:46]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-07-21 15:28:49]

Patchset 2 (id:??) landed as
https://crrev.com/e1ab6c84cbadc16f4d5d775e1ab3680fe0af2612
Cr-Commit-Position: refs/heads/master@{#339654}