[Anonymous Iframe] Change window.anonymous to window.isAnonymouslyFramed

Since there's a bunch of usage of window.anonymous in various of origins
currently in the world, we decide to change the naming to a more precise
and un-confusing `isAnonymouslyFramed`. See more:
WICG/anonymous-iframe#1

Bug: 1323180
Change-Id: I81637cdb004e87408235edb9b367062f62358577

html/anonymous-iframe/fenced-frame.tentative.https.window.js

@@ -32,8 +32,8 @@ promise_test(async test => {
 
   // 3. Expect it not to be considered anonymous.
   send(frame_fenced, `
-    send("${msg_queue}", window.anonymous);
+    send("${msg_queue}", window.isAnonymouslyFramed);
   `);
   assert_equals(await receive(msg_queue), "false",
-    "Check window.anonymous in FencedFrame");
+    "Check window.isAnonymouslyFramed in FencedFrame");
 }, 'FencedFrame within an AnonymousIframe is not anonymous')