Follow up cleanups for r222162.

Follow up cleanups for r222162.

Changes:
 - ScopedRefNPObject is wrapped into scoped_ptr<> when passed as a callback between threads since ScopedRefNPObject's ref-counting is not thread-safe.
 - Made HostNPScriptObject::DoGenerateKeyPair a static method to make bouncing between threads safe.
 - Switched the completion callbacks passed to the daemon controller and pairing registry to use a weak pointer to HostNPScriptObject.
 - Clarified threading semantic of DaemonController::Delegate.
 
BUG=156257, 173509
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=222397

[alexeypa (please no reviews), 2013-09-10 16:59:41]

PTAL.

[Wez, 2013-09-10 17:10:24]

LGTM, and thanks. :)

[commit-bot: I haz the power, 2013-09-10 17:44:35]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/alexeypa@chromium.org/23578017/1

[commit-bot: I haz the power, 2013-09-10 21:10:43]

Retried try job too often on linux_aura for step(s) browser_tests
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=linux_aura...

[commit-bot: I haz the power, 2013-09-10 22:03:53]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/alexeypa@chromium.org/23578017/1

[commit-bot: I haz the power, 2013-09-10 23:36:40]

Change committed as 222397