Revert of Change WokerThread to use a blink::WebThread (https://codereview.chromium.org/423303004/)

Reason for revert:
This change introduced leaks that were detected in the following layout tests:
fast/workers/worker-terminate.html ({"numberOfLiveDocuments":[1,2],"numberOfLiveNodes":[4,21]})
inspector-protocol/debugger/debugger-terminate-dedicated-worker-while-paused.html ({"numberOfLiveDocuments":[1,2],"numberOfLiveNodes":[4,32]})

http://build.chromium.org/p/chromium.webkit/builders/WebKit%20Linux%20Leak/builds/3599

Original issue's description:
> Change WokerThread to use a blink::WebThread
> 
> This is a reland of https://codereview.chromium.org/400153002/ in which I forgot the remove the changes made to WebThread for shared timer. The first patch set is equivalent to the previous CL.
> 
> BUG=301515
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=179217

TBR=jochen@chromium.org,nasko@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=301515

Review URL: https://codereview.chromium.org/432673003

git-svn-id: svn://svn.chromium.org/blink/trunk@179284 bbb929c8-8fbe-4397-9dbb-9b2b20218538