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

Relanding https://codereview.chromium.org/23242019/ now that Android build
busters are fixed on the Chromium-side.

BUG=272993
TEST=All existing media tests still pass.
TBR=adamk,jamesr

Review URL: https://chromiumcodereview.appspot.com/23171025

git-svn-id: svn://svn.chromium.org/blink/trunk@156653 bbb929c8-8fbe-4397-9dbb-9b2b20218538