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

Source/modules/mediasource/SourceBufferList.idl

Index: Source/modules/mediasource/SourceBufferList.idl
diff --git a/Source/modules/mediasource/WebKitSourceBufferList.idl b/Source/modules/mediasource/SourceBufferList.idl
similarity index 90%
copy from Source/modules/mediasource/WebKitSourceBufferList.idl
copy to Source/modules/mediasource/SourceBufferList.idl
index ba1768e44d16da585e89a5271b51319c18aec506..8c229657ccd96be40c5f6ad764a3ea51b38612f2 100644
--- a/Source/modules/mediasource/WebKitSourceBufferList.idl
+++ b/Source/modules/mediasource/SourceBufferList.idl
@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2012 Google Inc. All rights reserved.
+ * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
@@ -30,11 +30,12 @@
  
 [
     NoInterfaceObject,
-    EnabledAtRuntime=webKitMediaSource,
+    EnabledAtRuntime=mediaSource,
     EventTarget,
-] interface WebKitSourceBufferList {
+    CallWith=ScriptExecutionContext
+] interface SourceBufferList {
     readonly attribute unsigned long length;
-    getter WebKitSourceBuffer item(unsigned long index);
+    getter SourceBuffer item(unsigned long index);
 
     // EventTarget interface
     void addEventListener(DOMString type,