WindowAudioWorklet should use ContextLifecycleObserver instead of DOMWindowProperty

We're deprecating DOMWindowProperty. So this CL replaces it with ContextLifecycleObserver.

Also this CL adds a comment why WindowAudioWorklet needs to clear
m_audioWorklet in contextDestroyed().

BUG=610176

Review-Url: https://codereview.chromium.org/2567243005
Cr-Commit-Position: refs/heads/master@{#439068}
2 files changed