Remove WebMediaSourceClient interface and old WebMediaPlayer::load() methods.

BUG=272993
TEST=All existing media tests still pass.

Review URL: https://chromiumcodereview.appspot.com/23242019

git-svn-id: svn://svn.chromium.org/blink/trunk@156616 bbb929c8-8fbe-4397-9dbb-9b2b20218538