Add Maplike<> utility mixin class for implementing maplike interfaces

Source/modules/encryptedmedia/MediaKeyStatusMap.idl

Index: Source/modules/encryptedmedia/MediaKeyStatusMap.idl
diff --git a/Source/modules/encryptedmedia/MediaKeyStatusMap.idl b/Source/modules/encryptedmedia/MediaKeyStatusMap.idl
index 7a71c065baaaf74a9dd86c16084e524dc944f386..f0775839f6bfcf65a1909be51f43114cb5a642c5 100644
--- a/Source/modules/encryptedmedia/MediaKeyStatusMap.idl
+++ b/Source/modules/encryptedmedia/MediaKeyStatusMap.idl
@@ -15,10 +15,5 @@ enum MediaKeyStatus {
     GarbageCollected,
 ] interface MediaKeyStatusMap {
     readonly attribute unsigned long size;
-
-    // This function returns undefined if |!this->has(key)|.
-    [TreatReturnedNullStringAs=Undefined] MediaKeyStatus get(BufferSource key);
-    boolean has(BufferSource key);
-
     readonly maplike <BufferSource, MediaKeyStatus>;
 };