MSE: Remove old WebMediaSource::addSourceBuffer declaration

Now that both MediaSourceBase and Chromium WebMediaSourceImpl only call
or implement the version of WebMediaSource::addSourceBuffer() that
includes FrameProcessorChoice, this change removes the old version of
this method that did not have FrameProcessorChoice.

R=acolwell@chromium.org,abarth@chromium.org
BUG=249422
TEST=All MediaSource layout tests pass locally on Linux

Review URL: https://codereview.chromium.org/220123011

git-svn-id: svn://svn.chromium.org/blink/trunk@170796 bbb929c8-8fbe-4397-9dbb-9b2b20218538