fix canplaythrough

This makes canplaythrough follow the spec.
It fires when we estimate that we have enough data to play to the
end, or when we stop downloading because the buffer is full.

We do this by keeping a time series in BufferedDataSourceHostImpl
which is used to estimate the download speed over the last 10 seconds.

BUG=73609

Review-Url: https://codereview.chromium.org/2796193002
Cr-Commit-Position: refs/heads/master@{#469797}
8 files changed