Change blink::WebMediaPlayer::buffered() to return by-value

This removes the need for implementations to hand out references to mutable data.

Chromium change is https://codereview.chromium.org/282073003/.

BUG=360254

Review URL: https://codereview.chromium.org/284143002

git-svn-id: svn://svn.chromium.org/blink/trunk@174393 bbb929c8-8fbe-4397-9dbb-9b2b20218538