[MSE] Add a SourceBufferPrivateClient interface for platform -> html communication.

[MSE] Add a SourceBufferPrivateClient interface for platform -> html communication.
https://bugs.webkit.org/show_bug.cgi?id=123373

Reviewed by Eric Carlson.

To enable communication between SourceBuffer and SourceBufferPrivate without introducing
layering violations, add a new interface class SourceBufferPrivateInterface from which
SourceBuffer will inherit.

* Modules/mediasource/SourceBuffer.cpp:
(WebCore::SourceBuffer::SourceBuffer): Set the private's client.
(WebCore::SourceBuffer::~SourceBuffer): Clear the private's client.
(WebCore::SourceBuffer::sourceBufferPrivateDidEndStream): Added stub.
(WebCore::SourceBuffer::sourceBufferPrivateDidReceiveInitializationSegment): Hinno.
(WebCore::SourceBuffer::sourceBufferPrivateDidReceiveSample): Ditto.
(WebCore::SourceBuffer::sourceBufferPrivateHasAudio): Ditto.
(WebCore::SourceBuffer::sourceBufferPrivateHasVideo): Ditto.
* Modules/mediasource/SourceBuffer.h:
* WebCore.xcodeproj/project.pbxproj: Add new files to project.
* platform/graphics/SourceBufferPrivate.h:
* platform/graphics/SourceBufferPrivateClient.h: Added.
(WebCore::SourceBufferPrivateClient::~SourceBufferPrivateClient): Empty destructor.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@158606 268f45cc-cd09-0410-ab3c-d52691b4dbfc