Reland "[Longtasks] Fix buffering before observer"

This is a reland of c2d62d36. This is
safe to reland because it got reverted due to the issue that has been
fixed in https://bugs.chromium.org/p/chromium/issues/detail?id=1108676.

Original change's description:
> [Longtasks] Fix buffering before observer
>
> When we shipped longtasks with buffered flag, we forgot to remove a
> check that would prevent longtask creation when there's no observer.
> This CL fixes this and adds a test for this case. It also moves the
> UseCounter to the observe() method.
>
> Bug: 1131385
> Change-Id: I2911b9ab044db7394bcf64f64b1e599959cbdc37
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2463594
> Reviewed-by: Steve Kobes <skobes@chromium.org>
> Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#816664}

Bug: 1131385
Change-Id: I8640abc566fc7eceaf44e80c04d23ca59182542d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2491100Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819832}

third_party/blink/renderer/core/timing/performance.cc

@@ -675,10 +675,6 @@ void Performance::AddLongTaskTiming(base::TimeTicks start_time,
                                     const String& container_src,
                                     const String& container_id,
                                     const String& container_name) {
-  if (!HasObserverFor(PerformanceEntry::kLongTask))
-    return;
-
-  UseCounter::Count(GetExecutionContext(), WebFeature::kLongTaskObserver);
   auto* entry = MakeGarbageCollected<PerformanceLongTaskTiming>(
       MonotonicTimeToDOMHighResTimeStamp(start_time),
       MonotonicTimeToDOMHighResTimeStamp(end_time), name, container_type,

third_party/blink/renderer/core/timing/performance_observer.cc

@@ -211,6 +211,9 @@ void PerformanceObserver::observe(const PerformanceObserverInit* observer_init,
   if (filter_options_ & PerformanceEntry::kResource) {
     UseCounter::Count(GetExecutionContext(), WebFeature::kResourceTiming);
   }
+  if (filter_options_ & PerformanceEntry::kLongTask) {
+    UseCounter::Count(GetExecutionContext(), WebFeature::kLongTaskObserver);
+  }
   if (is_registered_)
     performance_->UpdatePerformanceObserverFilterOptions();
   else

third_party/blink/web_tests/external/wpt/longtask-timing/longtask-before-observer.window.js

+async_test(t => {
+  assert_implements(window.PerformanceLongTaskTiming, 'Longtasks are not supported.');
+  // Create a long task before any observer.
+  const begin = window.performance.now();
+  while (window.performance.now() < begin + 60);
+  // After a timeout, add an observer with buffered flag.
+  t.step_timeout(() => {
+    new PerformanceObserver(t.step_func_done(list => {
+      list.getEntries().forEach(entry => {
+        assert_equals(entry.entryType, 'longtask');
+        assert_equals(entry.name, 'self');
+        assert_greater_than(entry.duration, 50);
+      });
+    })).observe({type: 'longtask', buffered: true});
+  }, 0);
+}, 'PerformanceObserver with buffered flag can see previous longtask entries.');