[Longtasks] Ship the buffered flag

This change ships the buffered flag, moving some work to WindowPerformance to
allow unsubscribing when the buffer becomes full and there are no observers.
Intent:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/cX5ahS7nCFw

Bug: 1016815
Change-Id: I3c20c65ed921d4a939d2214b25d3c7154129adda
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1956075
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#723906}

longtask-timing/buffered-flag.window.js

@@ -8,7 +8,7 @@ async_test(t => {
                     return;
                 assert_equals(entry.entryType, 'longtask');
                 assert_equals(entry.name, 'self');
-                assert_greater_than(longtask.duration, 50);
+                assert_greater_than(entry.duration, 50);
                 longtaskObserved = true;
             });
             assert_true(longtaskObserved, 'Did not observe buffered longtask.');