Revert of Adds some CHECKs to MessagePumpMojo

Revert of Adds some CHECKs to MessagePumpMojo (https://codereview.chromium.org/454433003/)

Reason for revert:
Speculative revert, attempting to diagnose timing out Blink layout tests on Windows.

Original issue's description:
> Adds some CHECKs to MessagePumpMojo
> 
> The thread watcher appears to be kicking on when we're trying to
> remove a handle. Not sure why. This converts some DCHECKs to CHECKs
> and fixes handling of the deadline. I suspect none of this will help,
> but it's worth a shot.
> 
> BUG=399769
> TEST=none
> R=darin@chromium.org
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=288949

TBR=darin@chromium.org,sky@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=399769
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=289057

[adamk, 2014-08-12 20:32:35]

Created Revert of Adds some CHECKs to MessagePumpMojo

[commit-bot: I haz the power, 2014-08-12 20:38:29]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/adamk@chromium.org/461133002/1

[commit-bot: I haz the power, 2014-08-12 20:42:18]

Change committed as 289057

[Hajime Morrita, 2014-08-12 22:33:57]

FYI: The change seemed to causing a kind of busy loop on mojo-pump thread on
Linux.
The browser process consumes 100% cpu time with the original CL.