MSE: Implement SourceBuffer.trackDefaults

Source/modules/mediasource/SourceBuffer.idl

Index: Source/modules/mediasource/SourceBuffer.idl
diff --git a/Source/modules/mediasource/SourceBuffer.idl b/Source/modules/mediasource/SourceBuffer.idl
index 321daf7e491ea29a028c52def1826b0fd07a8fb1..395670eae0eed086d8a690ce47075e2d55f2c06a 100644
--- a/Source/modules/mediasource/SourceBuffer.idl
+++ b/Source/modules/mediasource/SourceBuffer.idl
@@ -69,5 +69,9 @@ enum AppendMode {
     // Abort the current segment append sequence.
     [RaisesException] void abort();
     [RaisesException] void remove(double start, unrestricted double end);
+
+    // Gets or sets the TrackDefaultList this SourceBuffer may consult during
+    // the initialization segment algorithm.
+    [RaisesException=Setter, RuntimeEnabled=MediaSourceExperimental] attribute TrackDefaultList trackDefaults;
 };