Check if dispatcher is destroyed during PreDispatch

During pre-dispatch phase of an event, the dispatcher might get
destroyed. We need to stop dispatching the event in that case.

BUG=412019

Review URL: https://codereview.chromium.org/1417033007

Cr-Commit-Position: refs/heads/master@{#358229}
3 files changed