Use Persistent<HeapHashSet<Member<Node>>> for gNodesDispatchingSimulatedClicks in EventDispatcher
Use Persistent<HeapHashSet<Member<Node>>> for gNodesDispatchingSimulatedClicks in EventDispatcher

Actually gNodesDispatchingSimulatedClicks doesn't have to be Persistent<HeapHashSet<Member<Node>>>, because the Nodes are guaranteed to exist on stack and thus their lifetime is already protected. However, it would be better to use Persistent<HeapHashSet<Member<Node>>> for clarity. I added a comment to the code about the lifetime.

BUG=340522

Review URL: https://codereview.chromium.org/381583002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@177745 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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