Expose native, desktop and mobile gesture detection defaults

ui/events/event.h

Index: ui/events/event.h
diff --git a/ui/events/event.h b/ui/events/event.h
index 57871944c0655c5e6e49c0fed482e13b979c89d1..98dd36cbda5b2cfb04a6bdba381d6369590f9a47 100644
--- a/ui/events/event.h
+++ b/ui/events/event.h
@@ -493,7 +493,8 @@ class EVENTS_EXPORT TouchEvent : public LocatedEvent {
         radius_x_(model.radius_x_),
         radius_y_(model.radius_y_),
         rotation_angle_(model.rotation_angle_),
-        force_(model.force_) {}
+        force_(model.force_),
+        may_cause_scrolling_(model.may_cause_scrolling_) {}
 
   TouchEvent(EventType type,
              const gfx::PointF& location,
@@ -521,6 +522,9 @@ class EVENTS_EXPORT TouchEvent : public LocatedEvent {
   float rotation_angle() const { return rotation_angle_; }
   float force() const { return force_; }
 
+  void set_may_cause_scrolling(bool causes) { may_cause_scrolling_ = causes; }

[jdduke (slow), 2014/12/11 19:06:38]

I'm open to different naming here, or event a different way of propagating this
bit.

+  bool may_cause_scrolling() const { return may_cause_scrolling_; }
+
   // Used for unit tests.
   void set_radius_x(const float r) { radius_x_ = r; }
   void set_radius_y(const float r) { radius_y_ = r; }
@@ -566,6 +570,10 @@ class EVENTS_EXPORT TouchEvent : public LocatedEvent {
 
   // Force (pressure) of the touch. Normalized to be [0, 1]. Default to be 0.0.
   float force_;
+
+  // Whether the (unhandled) touch event will produce a scroll event (i.e., it's
+  // a touchmove and exceeds the platform slop region). Defaults to false.
+  bool may_cause_scrolling_;

[jdduke (slow), 2014/12/11 19:32:54]

Awfully tempting to make this mutable... Would avoid the need to restructure
event-related APIs, and emphasizes that the value is likely to change without
affecting the core event, but it might be somewhat confusing...

 };
 
 // An interface that individual platforms can use to store additional data on