Remove MediaSourcePrivate/SourceBufferPrivate

These layers are just proxies for the Web* variants.

BUG=317353

Review URL: https://codereview.chromium.org/61603006

git-svn-id: svn://svn.chromium.org/blink/trunk@161859 bbb929c8-8fbe-4397-9dbb-9b2b20218538