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The hang monitor is supposed to wait for 30 seconds of inactivity before
deciding that a renderer is not responsive. This CL makes sure that when
waiting for multiple input events, receiving an ack is correctly interpreted
as a responsive renderer (for that moment)

BUG=336198

Review URL: https://codereview.chromium.org/535653002

Cr-Commit-Position: refs/heads/master@{#293060}