Use AutoThread in ChromotingHostContext & NPAPI plugin.

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

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=170360

[Wez, 2012-11-29 23:33:09]

FYI, this patch gives no errors under Windows TSan on my local box (I notice we
have no Win TSan try-bot, hint hint...); relanding. :)

[commit-bot: I haz the power, 2012-11-29 23:33:40]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/wez@chromium.org/11316247/2001

[commit-bot: I haz the power, 2012-11-30 02:16:22]

Change committed as 170360