Fix worker WebSocket crash caused by dereferencing weak pointer

WorkerWebSocketChannel::Peer::m_bridge is bound to workerGlobalScopeDidError
as |bridge|. Because it is a CrossThreadWeakPersistent, it's possible that
|m_bridge| is not null when bound but |bridge| is null when called. This CL
adds null pointer checks to avoid a crash.

BUG=625649

Review-Url: https://codereview.chromium.org/2123703002
Cr-Commit-Position: refs/heads/master@{#403744}
1 file changed