Found a few problems with the way the keyframe distance is calculated and
passed from the decoder to WMPI:
1. as soon as the video decoder is stopped we reset the average - meaning we
will stop considering optimizing playback for the video
2. we have to wait for more than 4 keyframes to start optimizing video
playback, this can take seconds
3. audio decoder resets the average to 0 in pipeline's shared state

BUG=None
TEST=Manual

Review-Url: https://codereview.chromium.org/2643103002
Cr-Commit-Position: refs/heads/master@{#444955}