Add minimal implementation of unprefixed MediaSource API that has feature parity with prefixed API

Source/modules/mediasource/SourceBuffer.idl

Index: Source/modules/mediasource/SourceBuffer.idl
diff --git a/Source/modules/webmidi/MIDIPort.idl b/Source/modules/mediasource/SourceBuffer.idl
similarity index 77%
copy from Source/modules/webmidi/MIDIPort.idl
copy to Source/modules/mediasource/SourceBuffer.idl
index e7e2aa9f87a204969318580f984b683adf0a657a..d04be7109d236ededb8eb49eebb87ece3aff57c7 100644
--- a/Source/modules/webmidi/MIDIPort.idl
+++ b/Source/modules/mediasource/SourceBuffer.idl
@@ -27,24 +27,28 @@
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
-
-enum MIDIPortType {
-    "input",
-    "output"
-};
-
+ 
 [
     NoInterfaceObject,
     ActiveDOMObject,
-    EventTarget
-] interface MIDIPort {
-    readonly attribute DOMString id;
-    readonly attribute DOMString manufacturer;
-    readonly attribute DOMString name;
-    readonly attribute MIDIPortType type;
-    readonly attribute DOMString version;
+    EventTarget,
+    EnabledAtRuntime=mediaSource
+] interface SourceBuffer {
+
+    readonly attribute boolean updating;
+  
+    // Returns the time ranges buffered.
+    [GetterRaisesException] readonly attribute TimeRanges buffered;
 
-    attribute EventListener ondisconnect;
+    // Applies an offset to media segment timestamps.
+    [SetterRaisesException] attribute double timestampOffset;
+
+    // Append segment data.
+    [RaisesException] void appendBuffer(ArrayBuffer data);
+    [RaisesException] void appendBuffer(ArrayBufferView data);
+
+    // Abort the current segment append sequence.
+    [RaisesException] void abort();
 
     // EventTarget interface
     void addEventListener(DOMString type,
@@ -55,3 +59,4 @@ enum MIDIPortType {
                              optional boolean useCapture);
     [RaisesException] boolean dispatchEvent(Event event);
 };
+