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 #13 id:240001 of https://codereview.chromium.org/1156503005/)

Reason for revert:
I suspect this CL breaks Windows 8 content_unittests, as it's listed both in
https://build.chromium.org/p/chromium.fyi/builders/Win8%20Tests%20%281%29/builds/8675 and our WebRTC-specific waterfall: https://build.chromium.org/p/chromium.webrtc/builders/Win8%20Tester/builds/14164

I'll try to reland if it turns out not to solve the breakage.

Original issue's description:
> Don't peek messages in the MessagePumpForUI class when we receive our kMsgHaveWork message.
>
> 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
> R=cpu
>
> Committed: https://crrev.com/b8e126c8b532b1327f38afe2bdf59aa5ff933971
> Cr-Commit-Position: refs/heads/master@{#333572}

TBR=cpu@chromium.org,jar@chromium.org,kbr@chromium.org,geofflang@chromium.org,scottmg@chromium.org,ananta@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=492837
Committed: https://crrev.com/e799035ef225f9238d5aa7bea125d6d0289db3cd
Cr-Commit-Position: refs/heads/master@{#333700}

[kjellander_chromium, 2015-06-10 07:53:25]

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