Bug 1577089 [wpt PR 18719] - Update interfaces/requestidlecallback.idl, a=testonly

Automatic update from web-platform-tests
Update interfaces/requestidlecallback.idl (#18719)

Source: https://github.com/tidoust/reffy-reports/blob/3eeb4d0/whatwg/idl/requestidlecallback.idl
Build: https://travis-ci.org/tidoust/reffy-reports/builds/577694005
--

wpt-commits: 8397686d52db575fd96fe17cd2b9c7ce30c4c10b
wpt-pr: 18719

testing/web-platform/tests/interfaces/requestidlecallback.idl

@@ -4,7 +4,7 @@
 // Source: Cooperative Scheduling of Background Tasks (https://w3c.github.io/requestidlecallback/)
 
 partial interface Window {
-  unsigned long requestIdleCallback(IdleRequestCallback callback, optional IdleRequestOptions options);
+  unsigned long requestIdleCallback(IdleRequestCallback callback, optional IdleRequestOptions options = {});
   void cancelIdleCallback(unsigned long handle);
 };