Drop [LegacyImplementedInBaseClass] from WindowMediaSource IDL interface

Source/modules/mediasource/WindowMediaSource.idl

Index: Source/modules/mediasource/WindowMediaSource.idl
diff --git a/Source/modules/mediasource/WindowMediaSource.idl b/Source/modules/mediasource/WindowMediaSource.idl
index b28e24317266126e6e9cf316a18e1a8bc6dc9cb8..ab65c2f5280132e3b9f929e04c463310599d7460 100644
--- a/Source/modules/mediasource/WindowMediaSource.idl
+++ b/Source/modules/mediasource/WindowMediaSource.idl
@@ -29,8 +29,7 @@
  */
 
 [
-    LegacyImplementedInBaseClass, // Actually no implementation, as just constructors; use to prevent a header from being included
-    RuntimeEnabled=WebKitMediaSource,
+    RuntimeEnabled=WebKitMediaSource
 ] partial interface Window {
     attribute WebKitMediaSourceConstructor WebKitMediaSource;
     attribute WebKitSourceBufferConstructor WebKitSourceBuffer;