Support 3f swipe for tab scrubbing

ui/base/events/event.h

Index: ui/base/events/event.h
diff --git a/ui/base/events/event.h b/ui/base/events/event.h
index 4f73146c84a45af3c0f5cf9432d5b20064ed7e8b..7d74fba634ab13a1da42df53161107de91445d1d 100644
--- a/ui/base/events/event.h
+++ b/ui/base/events/event.h
@@ -581,7 +581,8 @@ class UI_EXPORT ScrollEvent : public MouseEvent {
               int flags)
       : MouseEvent(model, source, target, type, flags),
         x_offset_(model.x_offset_),
-        y_offset_(model.y_offset_) {
+        y_offset_(model.y_offset_),
+        finger_count_(model.finger_count_){
   }
 
   // Used for tests.
@@ -593,10 +594,12 @@ class UI_EXPORT ScrollEvent : public MouseEvent {
 
   float x_offset() const { return x_offset_; }
   float y_offset() const { return y_offset_; }
+  int finger_count() const { return finger_count_; }
 
  private:
   float x_offset_;
   float y_offset_;
+  int finger_count_;
 
   DISALLOW_COPY_AND_ASSIGN(ScrollEvent);
 };