Add Maplike<> utility mixin class for implementing maplike interfaces

Source/modules/webmidi/MIDIInputMap.idl

Index: Source/modules/webmidi/MIDIInputMap.idl
diff --git a/Source/modules/webmidi/MIDIInputMap.idl b/Source/modules/webmidi/MIDIInputMap.idl
index fdbe87508909c1832d73fd2dbec154ab39e526eb..6ab5e7976101c9e4bdda9fbfc4e8ffcaa15f2755 100644
--- a/Source/modules/webmidi/MIDIInputMap.idl
+++ b/Source/modules/webmidi/MIDIInputMap.idl
@@ -10,8 +10,5 @@
     GarbageCollected,
 ] interface MIDIInputMap {
     readonly attribute unsigned long size;
-    // This function returns undefined if |!this->has(id)|.
-    [ImplementedAs=getForBinding, CallWith=ScriptState] any get(DOMString id);
-    boolean has(DOMString key);
     readonly maplike<DOMString, MIDIInput>;
 };