CookieStore: Update cookies attribute to be SameObject

This updates cookies attribute to be SameObject.
Further context & discussion on this can be found
here: https://github.com/WICG/cookie-store/pull/181#discussion_r489817744

Bug: 1129218
Change-Id: I7d17263b15bf06adaf7cc15e05c093a23c6aa035
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2416965
Commit-Queue: Victor Costan <pwnall@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808210}

third_party/blink/renderer/modules/cookie_store/service_worker_registration_cookies.idl

@@ -10,5 +10,5 @@
   RuntimeEnabled=CookieStoreWorker,
   ImplementedAs=ServiceWorkerRegistrationCookies
 ] partial interface ServiceWorkerRegistration {
-  readonly attribute CookieStoreManager cookies;
+  [SameObject] readonly attribute CookieStoreManager cookies;
 };