GSB uses delta_hints to calculate scrolling chain.

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

Index: third_party/WebKit/Source/core/page/scrolling/ScrollState.h
diff --git a/third_party/WebKit/Source/core/page/scrolling/ScrollState.h b/third_party/WebKit/Source/core/page/scrolling/ScrollState.h
index d9d893931269f3ef5c8a72e73cf7f12586e3e405..4660bd504205875acd7cda7e04b0a4e97648edd6 100644
--- a/third_party/WebKit/Source/core/page/scrolling/ScrollState.h
+++ b/third_party/WebKit/Source/core/page/scrolling/ScrollState.h
@@ -43,6 +43,12 @@ class CORE_EXPORT ScrollState final
   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; };