IDL: Add build target for IDL dictionary impl generation in core

Source/core/testing/TestingDictionary.idl

Index: Source/core/testing/TestingDictionary.idl
diff --git a/Source/core/testing/GarbageCollectedScriptWrappable.idl b/Source/core/testing/TestingDictionary.idl
similarity index 50%
copy from Source/core/testing/GarbageCollectedScriptWrappable.idl
copy to Source/core/testing/TestingDictionary.idl
index 0fd3bde6412d03694bd355dbf23cf278a2162ebe..bf16e4b61fb6cf11a77953330877cdfbb9b17ee2 100644
--- a/Source/core/testing/GarbageCollectedScriptWrappable.idl
+++ b/Source/core/testing/TestingDictionary.idl
@@ -4,6 +4,9 @@
 
 [
     GarbageCollected
-] interface GarbageCollectedScriptWrappable {
-    [NotEnumerable] DOMString toString();
+] dictionary TestingDictionary {
+    long longMember;
+    DOMString stringMember = "defaultStringValue";
+    boolean? booleanOrNullMember = null;
+    sequence<DOMString> stringSequenceMember;
 };