Revert of Don't peek messages in the MessagePumpForUI class when we receive our kMsgHaveWork message

Revert of Don't peek messages in the MessagePumpForUI class when we receive our kMsgHaveWork message. (patchset #1 id:1 of https://codereview.chromium.org/1173193002/)

Reason for revert:
Causing frequent wakeups (especially with watcher process?) and shows power regression on perf bots https://code.google.com/p/chromium/issues/detail?id=500749.

Potential fix at https://codereview.chromium.org/1189133003/

Original issue's description:
> Don't peek messages in the MessagePumpForUI class when we receive our kMsgHaveWork message.
>
> Relanding this patch with fixes for content_unittests failures. We need to wait for the UI worker thread to start.
>
> Currently the MessagePumpForUI class peeks Windows messages when we receive the kMsgHaveWork message in
> our main message loop and in nested modal loops. This is because the posted message starves the message loop
> of input and other lower priority messages. While this is ok for the main message loop our peeking and dispatching
> messages in nested loops is wrong and violates the silent contract where in the nested loop should be the one peeking
> and dispatching messages.
>
> To fix this the approach we are taking is to create a worker thread which uses a waitable timer of 3 ms which posts
> the kMsgHaveWork message to the main loop when the timer fires. As a result we can safely delete all the code
> in the MessagePumpForUI::ScheduleWork function and simplify the ProcessPumpReplacementMessage function.
>
> The MessagePumpForUI::ScheduleWork function now checks the delay for the task at the head of the queue
> If it is 0 then we post the message right away as it is a regular task. Added functions (GetNewlyAddedTaskDelay) to the MessagePump::Delegate class and the IncomingTaskQueue for this.
>
> The other change here is to change the GPU process to use the IO message pump by default and use the UI pump only
> if we are using opengl. Reason being this patch causes a delay in processing tasks due to the worker thread which
> causes tests like webgl_conformance to fail. We will continue working on addressing this over the coming days.
>
> BUG=492837
> TBR=cpu, scottmg
>
> Committed: https://crrev.com/29a20657bb92335c4e671cc1ee4c5e36ee1ffa1b
> Cr-Commit-Position: refs/heads/master@{#333831}

TBR=cpu@chromium.org,ananta@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=492837, 500749
Committed: https://crrev.com/4d306ecabfa83ed5e7ee61a97e30e96064bc100a
Cr-Commit-Position: refs/heads/master@{#335082}

[scottmg, 2015-06-18 18:14:43]

Created Revert of Don't peek messages in the MessagePumpForUI class when we
receive our kMsgHaveWork message.

[scottmg, 2015-06-18 18:14:53]

The CQ bit was unchecked by scottmg@chromium.org

[scottmg, 2015-06-18 18:15:25]

The CQ bit was checked by scottmg@chromium.org

[commit-bot: I haz the power, 2015-06-18 18:17:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1181263008/1

[commit-bot: I haz the power, 2015-06-18 18:21:48]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-18 18:22:28]

Patchset 1 (id:??) landed as
https://crrev.com/4d306ecabfa83ed5e7ee61a97e30e96064bc100a
Cr-Commit-Position: refs/heads/master@{#335082}