The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f433ac52f97edc48f644d69fb4d4488a2e74f680

commit f433ac52f97edc48f644d69fb4d4488a2e74f680
Author: Nicolás Peña Moreno <npm@chromium.org>
Date: Wed Oct 24 15:12:12 2018

[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/1296689
Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602342}
[modify] https://crrev.com/f433ac52f97edc48f644d69fb4d4488a2e74f680/third_party/blink/renderer/core/timing/event_timing.cc

Not closing this yet. Missing adding a layouttest for this, which someone else can do or I'll do it when I'm back.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d68fb50e144e2824951e07c72a3982efbd30d9fc

commit d68fb50e144e2824951e07c72a3982efbd30d9fc
Author: Nicolás Peña Moreno <npm@chromium.org>
Date: Thu Oct 25 06:58:14 2018

[EventTiming] Test only observing firstInput

This is a followup to commit
f433ac52f97edc48f644d69fb4d4488a2e74f680

It adds a test and optimizes by ignoring observers
when first input has already been detected.

Bug:  898484 

Change-Id: Ib4ed18538f16d5b292002d58114f16cc3337eee7
Reviewed-on: https://chromium-review.googlesource.com/c/1296554
Reviewed-by: Steve Kobes <skobes@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602627}
[add] https://crrev.com/d68fb50e144e2824951e07c72a3982efbd30d9fc/third_party/WebKit/LayoutTests/http/tests/event-timing/event-timing-only-observe-firstInput.html
[modify] https://crrev.com/d68fb50e144e2824951e07c72a3982efbd30d9fc/third_party/blink/renderer/core/timing/event_timing.cc
[modify] https://crrev.com/d68fb50e144e2824951e07c72a3982efbd30d9fc/third_party/blink/renderer/core/timing/window_performance.h