scheduler: Allow timer task to be nested

scheduler: Allow timer task to be nested

Blink's current shared timer mechanism, which we are trying to replace,
allows timers to execute in nested message loops, but only if the
nested message loop itself was not initiated by a timer. We believe
this limitation is a historical relic which was originally introduced
to prevent multiple invocations of the same timer[1].

When the shared timer is replaced by indivially posted tasks, the
underlying scheduler will ensure that each task is only executed once.
Therefore it should be safe to lift this restriction.

[1] https://trac.webkit.org/changeset/12774

BUG=463143, 416362, 480522
Committed: https://crrev.com/022f550d73963aa4ce245aeaa0515504d89b2e24
Cr-Commit-Position: refs/heads/master@{#331133}

[Sami, 2015-05-22 15:18:14]

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

[Sami, 2015-05-22 15:20:04]

PTAL. As a background we realized that the timer heap's nesting behavior is a
little more involved than just disallowing nesting completely. I'm hoping we can
ultimately remove all that special casing and just allow timers to run the same
way as any other task.

[alex clarke (OOO till 29th), 2015-05-22 15:27:15]

lgtm

[Sami, 2015-05-22 16:11:43]

The CQ bit was checked by skyostil@chromium.org

[commit-bot: I haz the power, 2015-05-22 16:12:02]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1153763005/1

[commit-bot: I haz the power, 2015-05-22 17:14:56]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-22 17:15:47]

Patchset 1 (id:??) landed as
https://crrev.com/022f550d73963aa4ce245aeaa0515504d89b2e24
Cr-Commit-Position: refs/heads/master@{#331133}