Add plumbing and support for crossfading StreamParserBuffers.

Per the MSE specification adds a fade-out section to StreamParserBuffer
which will contain buffers for crossfading.

Modifies SourceBufferStream::GetNextBuffer() to properly return these
buffers with a config change in between the fade out and fade in
sections.

BUG=334493
TEST=New unittests.
NOTRY=true

Review URL: https://codereview.chromium.org/125543002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245413 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245413