Move MediaSourceBase code into MediaSource.

Now that the prefixed MediaSource API is gone the MediaSourceBase class
is no longer needed. This change simply moves the code into the
MediaSource class.

Review URL: https://codereview.chromium.org/315723002

git-svn-id: svn://svn.chromium.org/blink/trunk@175407 bbb929c8-8fbe-4397-9dbb-9b2b20218538