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I was ambitious in r325585 to remove this behavior, asserting that the Port's
owner should clear its own listeners. This is an optimistic expectation, and a
backwards incompatible one at any rate.

BUG=486715
R=rockot@chromium.org

Review URL: https://codereview.chromium.org/1135123002

Cr-Commit-Position: refs/heads/master@{#329265}