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    to receive the ack on browser/.

We perform a sanity check upon background page ACKing an event,
    by checking whether the ACKed message is expected to
    be ACKed. We store the pending ACK message IDs into a set
    |unacked_messages_|, but due to PostTask call to
    IncrementInFlightEventsOnUI, the event ack might arrive earlier
    than storing the pending message's ID. This results in incorectly
    kill the renderer process.

BUG=485440, 473821
Test=Rise Vision chrome app shouldn't crash anymore.

Review URL: https://codereview.chromium.org/1138643010

Cr-Commit-Position: refs/heads/master@{#330301}