Remove a frame of delay on main-thread FlingStart. [2nd land]

third_party/WebKit/Source/web/WebViewImpl.cpp

 /*
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  *
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@@ skipping 681 matching lines @@
       fling_modifier_ = event.GetModifiers();
       fling_source_device_ = event.source_device;
       DCHECK_NE(fling_source_device_, kWebGestureDeviceUninitialized);
       std::unique_ptr<WebGestureCurve> fling_curve =
           Platform::Current()->CreateFlingAnimationCurve(
               event.source_device,
               WebFloatPoint(event.data.fling_start.velocity_x,
                             event.data.fling_start.velocity_y),
               WebSize());
       DCHECK(fling_curve);
-      gesture_animation_ = WebActiveGestureAnimation::CreateAtAnimationStart(

[dtapuska, 2017/05/31 00:53:11]

Do we want this to be the event creation time or Now()?

It seems that if we have a janky main thread you could end up missing a few
frames of the fling animation. Is that anticipated?

[aelias_OOO_until_Jul13, 2017/05/31 01:07:06]

My rule of thumb is that in cases of jank during animations, it's best to
eventually recover to the same position as if the jank had never happened
(preserving animation rate).  That argues for using the event timestamp.  This
is consistent with what happens if the main thread is janky after the animation
has already started, and it's also consistent with what impl-thread FlingStart
does (it uses event timestamp, not Now).

-          std::move(fling_curve), this);
+      gesture_animation_ = WebActiveGestureAnimation::CreateWithTimeOffset(
+          std::move(fling_curve), this, event.TimeStampSeconds());
       MainFrameImpl()->FrameWidget()->ScheduleAnimation();
       event_result = WebInputEventResult::kHandledSystem;
 
       WebGestureEvent scaled_event =
           TransformWebGestureEvent(MainFrameImpl()->GetFrameView(), event);
       // Plugins may need to see GestureFlingStart to balance
       // GestureScrollBegin (since the former replaces GestureScrollEnd when
       // transitioning to a fling).
       // TODO(dtapuska): Why isn't the response used?
       MainFrameImpl()->GetFrame()->GetEventHandler().HandleGestureScrollEvent(
@@ skipping 3437 matching lines @@
   if (focused_frame->LocalFrameRoot() != MainFrameImpl()->GetFrame())
     return nullptr;
   return focused_frame;
 }
 
 LocalFrame* WebViewImpl::FocusedLocalFrameAvailableForIme() const {
   return ime_accept_events_ ? FocusedLocalFrameInWidget() : nullptr;
 }
 
 }  // namespace blink