Revert 170360

Revert 170360
> Use AutoThread in ChromotingHostContext & NPAPI plugin.
> 
> Callers now create ChromotingHostContext to create a set of threads
> for host tasks to run on, and pass the threads' TaskRunners to each
> host component explicitly. The ChromotingHostContext can then be
> torn down as soon as the caller no longer needs to create new
> components using the threads, and the threads themselves will exit
> only when the created components no longer require them.
> 
> This is a re-land of 11094056, which failed on the valgrind & Windows TSan bots due to ChromotingHostContext.StartAndStop not running its message loop to allow the context's threads join tasks to execute. 
> 
> TBR=alexeypa
> 
> Review URL: https://chromiumcodereview.appspot.com/11316247

TBR=wez@chromium.org
BUG=163702
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=170591