Facilitates gapless playback across mp3 and aac. Relying on splice
frames and crossfading doesn't work in cases where the prior segment
signal diverges to null-padding values too quickly relative to the
newly appended segment.
BUG=395899
TEST=new unittests. llama-demo works.
Review URL: https://codereview.chromium.org/414603002
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@287174 0039d316-1c4b-4281-b951-d872f2087c98