Revert 211956 "base: Change WeakPtr to use SequenceChecker inste..."

Revert 211956 "base: Change WeakPtr to use SequenceChecker inste..." 
As well as revert 212725 "base: Make SequenceChecker death tests multi-threads work correctly." 

See http://crbug.com/261448 

> base: Change WeakPtr to use SequenceChecker instead of ThreadChecker. 
> 
> This will enable WeakPtr to be used in SequencedWorkerPool, et al. with a sequence token. 
> 
> This is a continuation of issue: https://chromiumcodereview.appspot.com/18231002/ 
> 
> The original issue got messed up by a rietveld bug, so refer there for history and comments. 
> 
> BUG=165590
> 
> Review URL: https://chromiumcodereview.appspot.com/18501008 

TBR=tommycli@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=212780

[commit-bot: I haz the power, 2013-07-20 06:13:56]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/vandebo@chromium.org/19695005/1

[commit-bot: I haz the power, 2013-07-20 06:45:07]

Step "update" is always a major failure.
Look at the try server FAQ for more details.
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=win_rel&nu...

[commit-bot: I haz the power, 2013-07-20 07:08:08]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/vandebo@chromium.org/19695005/9003

[commit-bot: I haz the power, 2013-07-20 11:51:30]

Change committed as 212780