GSB uses delta_hints to calculate scrolling chain.

third_party/WebKit/public/platform/WebGestureEvent.h

Index: third_party/WebKit/public/platform/WebGestureEvent.h
diff --git a/third_party/WebKit/public/platform/WebGestureEvent.h b/third_party/WebKit/public/platform/WebGestureEvent.h
index 94d56575b0612378ded98ae89cf2e75763b73e5c..9bd89255998e0ad32a909a83631648dc020fdfd0 100644
--- a/third_party/WebKit/public/platform/WebGestureEvent.h
+++ b/third_party/WebKit/public/platform/WebGestureEvent.h
@@ -85,9 +85,9 @@ class WebGestureEvent : public WebInputEvent {
 
     struct {
       // Initial motion that triggered the scroll.
-      // May be redundant with deltaX/deltaY in the first scrollUpdate.
       float delta_x_hint;
       float delta_y_hint;
+      bool ignore_delta_hints;

[tdresser, 2017/06/23 19:14:12]

Why do we need this? Should it have a comment?

[sahel, 2017/06/23 20:01:00]

As you mentioned in previous comments, relying on zero delta hints is enough. I
deleted the redundant parameter.

       // Default initialized to ScrollUnits::PrecisePixels.
       ScrollUnits delta_hint_units;
       // If true, this event will skip hit testing to find a scroll