Keep ScriptProcessorHandler alive across threads

When posting a task from the ScriptProcessorHandler::Process to fire a
process event, we need to keep the handler alive in case the
ScriptProcessorNode goes away (because it has no onaudioprocess
handler) and removes the its handler.

Bug: 765495
Test: 
Change-Id: Ib4fa39d7b112c7051897700a1eff9f59a4a7a054
Reviewed-on: https://chromium-review.googlesource.com/677137
Reviewed-by: Hongchan Choi <hongchan@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Raymond Toy <rtoy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#503629}