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.

TBR=rtoy@chromium.org

(cherry picked from commit 783c28d59c4c748ef9b787d4717882c90c5b227b)

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-Original-Commit-Position: refs/heads/master@{#503629}
Reviewed-on: https://chromium-review.googlesource.com/679266
Reviewed-by: Raymond Toy <rtoy@chromium.org>
Cr-Commit-Position: refs/branch-heads/3202@{#403}
Cr-Branched-From: fa6a5d87adff761bc16afc5498c3f5944c1daa68-refs/heads/master@{#499098}
1 file changed