Not counting nested loops as time spent in a task

Not counting nested loops as time spent in a task.

While in a nested loop, Chrome is not blocked, keeps updating the client area and is responding to user actions.
Subtracting the time in nested loops from the task in which it happens, so that we don't diagnose this task as a source of jank.

BUG=401560, 440919
Committed: https://crrev.com/86708713f8a40a9155d56310b9b9a63515f58846
Cr-Commit-Position: refs/heads/master@{#308218}

[vadimt, 2014-12-12 22:19:34]

vadimt@chromium.org changed reviewers:
+ sky@chromium.org

[vadimt, 2014-12-12 22:19:35]

sky@, please review.

[sky, 2014-12-13 00:17:40]

LGTM

[vadimt, 2014-12-13 00:19:38]

The CQ bit was checked by vadimt@chromium.org

[commit-bot: I haz the power, 2014-12-13 00:22:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/788883004/1

[commit-bot: I haz the power, 2014-12-13 01:52:23]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-12-13 01:53:05]

Patchset 1 (id:??) landed as
https://crrev.com/86708713f8a40a9155d56310b9b9a63515f58846
Cr-Commit-Position: refs/heads/master@{#308218}