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

This reverts commit f8bfd726649508eaafde70b8353874e8cef501a9.

It broke Android build.

> ../../content/renderer/media/android/webmediaplayer_android.cc:30:58: fatalerror: third_party/WebKit/public/web/WebMediaSource.h: No such file or directory
> ../../content/renderer/media/android/media_source_delegate.cc:19:58: fatalerror: third_party/WebKit/public/web/WebMediaSource.h: No such file or directory

TBR=acolwell@chromium.org
BUG=

Review URL: https://codereview.chromium.org/23241024

git-svn-id: svn://svn.chromium.org/blink/trunk@156618 bbb929c8-8fbe-4397-9dbb-9b2b20218538