GSB uses delta_hints to calculate scrolling chain.

third_party/WebKit/Source/core/page/scrolling/ScrollState.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef ScrollState_h
 #define ScrollState_h
 
 #include <deque>
 #include <memory>
 #include "bindings/core/v8/ExceptionState.h"
@@ skipping 25 matching lines @@
   void consumeDelta(double x, double y, ExceptionState&);
   // Pops the first element off of |m_scrollChain| and calls |distributeScroll|
   // on it.
   void distributeToScrollChainDescendant();
   int positionX() { return data_->position_x; };
   int positionY() { return data_->position_y; };
   // Positive when scrolling right.
   double deltaX() const { return data_->delta_x; };
   // Positive when scrolling down.
   double deltaY() const { return data_->delta_y; };
+  // Positive when scrolling right.
+  double deltaXHint() const { return data_->delta_x_hint; };
+  // Positive when scrolling down.
+  double deltaYHint() const { return data_->delta_y_hint; };
+  // Indicates whether delta hints should be ignored while handling GSB or not.

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

When this work is done, is this needed, or not? Could we just use 0 values for
the deltas if this isn't needed?

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

You are right, I initially added this for doing two rounds of GSB hittesting
once with scroll deltas and if the result cannot consume then with ignoring
delta hints. After talking to bokan@, dtapuska@ we decided that it's not needed.

For the sake of event_sender treating zero delta hints as true is enough. I
deleted the parameter.

+  bool ignoreDeltaHints() const { return data_->ignore_delta_hints; };
   // Indicates the smallest delta the input device can produce. 0 for
   // unquantized inputs.
   double deltaGranularity() const { return data_->delta_granularity; };
   // Positive if moving right.
   double velocityX() const { return data_->velocity_x; };
   // Positive if moving down.
   double velocityY() const { return data_->velocity_y; };
   // True for events dispatched after the users's gesture has finished.
   bool inInertialPhase() const { return data_->is_in_inertial_phase; };
   // True if this is the first event for this scroll.
@@ skipping 39 matching lines @@
   explicit ScrollState(std::unique_ptr<ScrollStateData>);
 
   std::unique_ptr<ScrollStateData> data_;
   std::deque<int> scroll_chain_;
   Member<Element> element_;
 };
 
 }  // namespace blink
 
 #endif  // ScrollState_h