Simplify WorkerGlobalScope::m_eventListeners.

Switch to using a 'mere' HashSet<> for these event listeners. Mainly
done to diagnose GC instability, but removing the use of an involved
abstraction like HeapListHashSet<> is something wanted regardless.

R=
BUG=677654

Review-Url: https://codereview.chromium.org/2609413002
Cr-Commit-Position: refs/heads/master@{#441364}
2 files changed