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..2b066d56188c5a7ae999492165d8e74b81b8734f 100644
--- a/ui/events/gesture_event_details.h
+++ b/ui/events/gesture_event_details.h
@@ -17,9 +17,22 @@ 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.

[sky, 2017/02/17 22:52:31]

Are you sure this isn't DIPs and what you are calling PRECISE_PIXELS is pixels?

[chengx, 2017/02/17 23:19:18]

I agree the names here, which originate from WebGestureEvent.h, is confusing.
Here is the thing. We didn't have ScrollUnits here for GestureEventDetails, so
when GestureEventDetails was used to create WebGestureEvent in
blink_event_util.cc, as in 
https://cs.chromium.org/chromium/src/ui/events/blink/blink_event_util.cc?type...
WebGestureEvent used the default value for the scroll units. The default value
is PrecisePixels, as documented in
https://cs.chromium.org/chromium/src/third_party/WebKit/public/platform/WebGe...
and
https://cs.chromium.org/chromium/src/third_party/WebKit/public/platform/WebGe...

So I think using PRECISE_PIXELS (same as PrecisePixels in WebGestureEvent.h just
using ALL_CAPS) here is fine. Besides, I tried PIXELS as default and some try
bots failed, which is expected because of inconsistency.

The names in the enum here are consistent with the ones in WebGestureEvent.h,
which have been used in a lot of places throughout chrome. If you think we
should rename them, probably we need to do it in another CL. 

+    PAGE                 // page (visible viewport) based scrolling.
+  };
+
   GestureEventDetails();
   explicit GestureEventDetails(EventType type);
-  GestureEventDetails(EventType type, float delta_x, float delta_y);
+
+  // ScrollUnits::PRECISE_PIXELS is the default in WebGestureEvent.h too.
+  GestureEventDetails(EventType type,
+                      float delta_x,
+                      float delta_y,
+                      ScrollUnits units = ScrollUnits::PRECISE_PIXELS);
 
   // 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 +71,11 @@ 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 +86,11 @@ 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 +185,13 @@ 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;