Enable spec-compliant canplaythrough event by default

The results from this study are in. As expected, Media.Video.Autoplay.Attribute.WaitTime goes up, by up to 10%. In return we get a bigger than expected positive impact on Media.UnderflowDuration and Media.MeanTimeBetweenRebuffers.AudioVideo.SRC.
Going to default this feature to ON, but leave the flag in place for one release in case we need to revert it for any reason.

BUG=725576, 73609

Review-Url: https://codereview.chromium.org/2932553003
Cr-Commit-Position: refs/heads/master@{#478093}
1 file changed