Promptly release cross-thread websocket bridge/proxy references

To prevent other threads using cross-thread persistents from accessing
to-be-finalized websocket Peer and Bridge abstractions, promptly free
them on becoming garbage. Delaying this until lazy sweeping runs risks
(benignly) touching poisoned objects.

R=
BUG=619373

Review-Url: https://codereview.chromium.org/2064633002
Cr-Commit-Position: refs/heads/master@{#399442}
1 file changed