Currently we limit preloading to 20Mb. This can be a problem with high bitrate
videos which mux a lot of data per chunk. Usually our goal is to preload 10 seconds
of data, but in some examples, only 6 seconds is preloaded because of this limit.

This CL increases the max buffer limit to 50Mb. This should only impact videos with
average bitrates over 16Mbit/s. Videos with bitrates up to 41Mbit/s will have 10 seconds
of preloading.

Also simplify the buffering calculations a bit by getting rid of some opportunistic pinning behind the curring reading position.

Finally, add some tests.

BUG=512993

Review-Url: https://codereview.chromium.org/2477513003
Cr-Commit-Position: refs/heads/master@{#430022}