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The delay agnostic AEC uses a signal based delay correction method to adjust buffer synchronization between loudspeaker and microphone. On Mac in particular we have seen deviations in UMA stats that point towards an echo already at startup. This is likely due to an early and incorrect correction based on poor audio data. By waiting 15 seconds before we turn on the ability to correct we can avoid a majority of these. The reported delay values are in general accurate enough and relying on them in the beginning is fine. The value 15 seconds is chosen because we have seen from UMA data that a significant amount of calls tend to end before 15 seconds when being in the UseDelayAgnosticAEC Finch experiment. We turn this "feature" on for all platforms but Android, where the reported system delays are inaccurate and we want to take action as soon as possible. In addition, the set of "good" delay values has been increased from 25% to 75% of the filter length. BUG=webrtc:3504 R=henrik.lundin@webrtc.org Review URL: https://webrtc-codereview.appspot.com/50199004 Cr-Commit-Position: refs/heads/master@{#9376}