[EventTiming] Allow only observing first input

This CL adds a check needed to properly produce first input
events when the PerformanceObserver is only observing
'firstInput' but not 'event'.

Bug: 898484

Change-Id: I8331b0c174a51db65a9388d67054dfbf5a7ecfba
Reviewed-on: https://chromium-review.googlesource.com/c/1296689Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602342}

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

@@ -37,7 +37,8 @@ void EventTiming::WillDispatchEvent(const Event& event) {
   // dispatched.
   if ((performance_->ShouldBufferEventTiming() &&
        !performance_->IsEventTimingBufferFull()) ||
-      performance_->HasObserverFor(PerformanceEntry::kEvent)) {
+      performance_->HasObserverFor(PerformanceEntry::kEvent)
+      || performance_->HasObserverFor(PerformanceEntry::kFirstInput)) {
     processing_start_ = CurrentTimeTicks();
     finished_will_dispatch_event_ = true;
   }