Add toplevel trace event to MessagePumpLibevent::OnLibeventNotification

Add toplevel trace event to MessagePumpLibevent::OnLibeventNotification

This adds a new top level trace span for work resulting from I/O
notifications. Currently I/O triggered work may not show up at all in
tracing (unlike work resulting from PostTask).

BUG=none
TEST=chrome://tracing on Chromebook Pixel. Input thread shows I/O spans
  rather than showing nothing.

Committed: https://crrev.com/da537fb071ac3c6be575afa410895a2838af0f66
Cr-Commit-Position: refs/heads/master@{#326855}

[spang, 2015-04-21 17:11:58]

spang@chromium.org changed reviewers:
+ danakj@chromium.org, dsinclair@chromium.org

[danakj, 2015-04-24 18:09:36]

LGTM

[spang, 2015-04-24 18:22:54]

The CQ bit was checked by spang@chromium.org

[commit-bot: I haz the power, 2015-04-24 18:23:57]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1097283004/1

[commit-bot: I haz the power, 2015-04-24 19:26:42]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-24 19:27:32]

Patchset 1 (id:??) landed as
https://crrev.com/da537fb071ac3c6be575afa410895a2838af0f66
Cr-Commit-Position: refs/heads/master@{#326855}