[worklets] Change the default credentials mode from 'omit' to 'same-origin' (w3c#757)

Issue: w3c#756

Author: nhiroki

worklets/Overview.bs

@@ -329,7 +329,7 @@ interface Worklet {
 };
 
 dictionary WorkletOptions {
-    RequestCredentials credentials = "omit";
+    RequestCredentials credentials = "same-origin";
 };
 </pre>