Fix mouse wheel over-scrolls when display is scaled and scroll is paginated

ui/events/gesture_event_details.h

Index: ui/events/gesture_event_details.h
diff --git a/ui/events/gesture_event_details.h b/ui/events/gesture_event_details.h
index 377c4232c991e3c0d580002421667ce311b69bfa..12692778a12ee02630c84c9748c32461e2015921 100644
--- a/ui/events/gesture_event_details.h
+++ b/ui/events/gesture_event_details.h
@@ -17,9 +17,20 @@ namespace ui {
 
 struct EVENTS_BASE_EXPORT GestureEventDetails {
  public:
+  // The definition of ScrollUnits below is consistent with that in
+  // //src/third_party/WebKit/public/platform/WebGestureEvent.h
+  enum class ScrollUnits {
+    PRECISE_PIXELS = 0,  // generated by high precision devices.
+    PIXELS,              // large pixel jump duration; should animate to delta.
+    PAGE                 // page (visible viewport) based scrolling.
+  };
+
   GestureEventDetails();
   explicit GestureEventDetails(EventType type);
-  GestureEventDetails(EventType type, float delta_x, float delta_y);
+  GestureEventDetails(EventType type,
+                      float delta_x,
+                      float delta_y,
+                      ScrollUnits units = ScrollUnits::PRECISE_PIXELS);

[Bret, 2017/02/17 21:06:03]

nit: add a short comment saying this is the default in WebGestureEvent too

[chengx, 2017/02/17 21:57:45]

Done.

 
   // The caller is responsible for ensuring that the gesture data from |other|
   // is compatible and sufficient for that expected by gestures of |type|.
@@ -58,6 +69,11 @@ struct EVENTS_BASE_EXPORT GestureEventDetails {
     return data_.scroll_begin.y_hint;
   }
 
+  ScrollUnits scroll_begin_units() const {
+    DCHECK_EQ(ET_GESTURE_SCROLL_BEGIN, type_);
+    return data_.scroll_begin.delta_hint_units;
+  }
+
   float scroll_x() const {
     DCHECK_EQ(ET_GESTURE_SCROLL_UPDATE, type_);
     return data_.scroll_update.x;
@@ -68,6 +84,11 @@ struct EVENTS_BASE_EXPORT GestureEventDetails {
     return data_.scroll_update.y;
   }
 
+  ScrollUnits scroll_update_units() const {
+    DCHECK_EQ(ET_GESTURE_SCROLL_UPDATE, type_);
+    return data_.scroll_update.delta_units;
+  }
+
   float velocity_x() const {
     DCHECK_EQ(ET_SCROLL_FLING_START, type_);
     return data_.fling_velocity.x;
@@ -162,11 +183,13 @@ struct EVENTS_BASE_EXPORT GestureEventDetails {
       // the x/y values from the first scroll_update.
       float x_hint;
       float y_hint;
+      ScrollUnits delta_hint_units;
     } scroll_begin;
 
     struct {  // SCROLL delta.
       float x;
       float y;
+      ScrollUnits delta_units;
       // Whether any previous scroll update in the current scroll sequence was
       // suppressed because the underlying touch was consumed.
       bool previous_update_in_sequence_prevented;