The helper functions for slop region check and subtraction are modified.

ui/events/gesture_detection/gesture_provider_unittest.cc

Index: ui/events/gesture_detection/gesture_provider_unittest.cc
diff --git a/ui/events/gesture_detection/gesture_provider_unittest.cc b/ui/events/gesture_detection/gesture_provider_unittest.cc
index 6be69847501c4a661da6f29abe2444b99e7ca19d..de81ff400c3fb5f14765b554998c834b0c6c306a 100644
--- a/ui/events/gesture_detection/gesture_provider_unittest.cc
+++ b/ui/events/gesture_detection/gesture_provider_unittest.cc
@@ -1064,6 +1064,39 @@ TEST_F(GestureProviderTest, NoSlopRegionCheckOnThreeFingerScroll) {
   EXPECT_EQ(3U, GetReceivedGestureCount());
 }
 
+TEST_F(GestureProviderTest, ScrollStartWithSecondaryPointer) {

[tdresser, 2016/07/22 15:57:45]

Out of curiosity, what ended up being the solution to your previous problem
here?

[sahel, 2016/07/22 19:46:42]

Creating events with the sequence of action_down, action_pointer_down,
action_pointer_up (even with setting the action index to be the first pointer),
didn't help.

Because each time new events were created and the ids would be set
appropriately. 

I used utility functions on the same event instead.
I also had to add a new utility function that can release a pointer with a given
index rather than always the last pointer.

+  EnableTwoFingerTap(kMaxTwoFingerTapSeparation, base::TimeDelta());
+  const float scaled_touch_slop = GetTouchSlop();
+
+  base::TimeTicks event_time = base::TimeTicks::Now();
+
+  MockMotionEvent event =
+      ObtainMotionEvent(event_time, MotionEvent::ACTION_DOWN, 0, 0);
+  EXPECT_TRUE(gesture_provider_->OnTouchEvent(event));
+
+  // Wait for the two finger tap timeout, and put the second pointer down.
+  event_time += kOneSecond;
+  event = ObtainMotionEvent(event_time, MotionEvent::ACTION_POINTER_DOWN, 0, 0,
+                            kMaxTwoFingerTapSeparation / 2, 0);
+  EXPECT_TRUE(gesture_provider_->OnTouchEvent(event));
+
+  event.ReleasePointAt(0);
+  EXPECT_TRUE(gesture_provider_->OnTouchEvent(event));
+
+  event.set_action(MotionEvent::ACTION_MOVE);
+  event.MovePoint(0, kMaxTwoFingerTapSeparation / 2, 2 * scaled_touch_slop);
+  EXPECT_TRUE(gesture_provider_->OnTouchEvent(event));
+
+  event.ReleasePoint();
+  EXPECT_TRUE(gesture_provider_->OnTouchEvent(event));
+
+  EXPECT_EQ(ET_GESTURE_TAP_DOWN, GetReceivedGesture(0).type());
+  EXPECT_EQ(ET_GESTURE_SCROLL_BEGIN, GetReceivedGesture(1).type());
+  EXPECT_EQ(ET_GESTURE_SCROLL_UPDATE, GetReceivedGesture(2).type());
+  EXPECT_EQ(ET_GESTURE_SCROLL_END, GetReceivedGesture(3).type());
+  EXPECT_EQ(4U, GetReceivedGestureCount());
+}
+
 TEST_F(GestureProviderTest, LongPressAndTapCancelledWhenScrollBegins) {
   base::TimeTicks event_time = base::TimeTicks::Now();