cc: Add trace events to track main thread latency mode

cc: Add trace events to track main thread latency mode

This will make easy to tell when the main thread is
in a high latency mode without having to infer based
on the surrounding context.

BUG=none

Committed: https://crrev.com/bca35e1882f448b92b0d79e2ce786629db96adec
Cr-Commit-Position: refs/heads/master@{#297110}

[brianderson, 2014-09-10 22:54:51]

brianderson@chromium.org changed reviewers:
+ jdduke@chromium.org, skyostil@chromium.org

[brianderson, 2014-09-10 22:54:52]

ptal.

Jared was asking for something like this earlier.

[Sami, 2014-09-11 11:36:02]

lgtm, looks useful!

Just an idea, but another option would be to add a trace counter that shows the
value of main_thread_in_high_latency_mode over time. That one should probably be
disabled by default since right now counters take up a lot of space.

[brianderson, 2014-09-27 05:36:41]

The CQ bit was checked by brianderson@chromium.org

[commit-bot: I haz the power, 2014-09-27 05:37:17]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/561043002/20001

[commit-bot: I haz the power, 2014-09-27 08:00:17]

Committed patchset #2 (id:20001) as 2b3a54932d4d5c0a0bc67ab357b833d28f1e92f7

[commit-bot: I haz the power, 2014-09-27 08:00:49]

Patchset 2 (id:??) landed as
https://crrev.com/bca35e1882f448b92b0d79e2ce786629db96adec
Cr-Commit-Position: refs/heads/master@{#297110}