Revert of [scheduler]: Fix a bug with non-nestable tasks.

Revert of [scheduler]: Fix a bug with non-nestable tasks. (patchset #2 id:20001 of https://codereview.chromium.org/1287003003/ )

Reason for revert:
Causing crashes on Mac.

BUG=520524
Original issue's description:
> [scheduler]: Fix a bug with non-nestable tasks.
>
> Avoids a DCHECK in TaskQueueImpl::Task::enqueue_order() caused by
> TaskIsOlderThanQueuedTasks being called on an empty task if the previous
> task was a non-nestable task while the scheduler is running on a nested
> message loop.
>
> Committed: https://crrev.com/c0d31ab28e21c048f40584ace3b0a8aca5723c17
> Cr-Commit-Position: refs/heads/master@{#343026}

TBR=skyostil@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Committed: https://crrev.com/8a68a446db4553aad0f31e68baef0ae638b92f7f
Cr-Commit-Position: refs/heads/master@{#343194}

[rmcilroy, 2015-08-13 14:54:52]

Created Revert of [scheduler]: Fix a bug with non-nestable tasks.

[commit-bot: I haz the power, 2015-08-13 14:54:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1287333002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1287333002/1

[commit-bot: I haz the power, 2015-08-13 14:55:22]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-13 14:56:00]

Patchset 1 (id:??) landed as
https://crrev.com/8a68a446db4553aad0f31e68baef0ae638b92f7f
Cr-Commit-Position: refs/heads/master@{#343194}