Notify Blink about start of gesture scroll through a queued event.

third_party/WebKit/LayoutTests/fast/events/pointerevents/touch-pointercancel.html

Index: third_party/WebKit/LayoutTests/fast/events/pointerevents/touch-pointercancel.html
diff --git a/third_party/WebKit/LayoutTests/fast/events/pointerevents/touch-pointercancel.html b/third_party/WebKit/LayoutTests/fast/events/pointerevents/touch-pointercancel.html
index 7ab34becb5c7f89bafbab6b423efd93c17ad4714..fda336097973ff5154523ae76f0399f629fae6df 100644
--- a/third_party/WebKit/LayoutTests/fast/events/pointerevents/touch-pointercancel.html
+++ b/third_party/WebKit/LayoutTests/fast/events/pointerevents/touch-pointercancel.html
@@ -61,10 +61,10 @@ function testSingleDrag(uncanceledMoveCausesScroll) {
   eventSender.touchMove();
 
   eventSender.updateTouchPoint(0, x, y+100);
+  eventSender.touchMove();

[tdresser, 2016/04/01 14:50:24]

Why do we only notify after this touchMove, instead of the previous one?

[mustaq, 2016/04/01 18:31:04]

I originally put it here to emphasize that pointercancel firing is triggered
specifically by touchMoveCausingScrollIfUncanceled and not by any side-effect of
touch-events. The same argument now applies to notifyStartOfTouchScroll.

If you still prefer the ideal sequence here, I will move the notification a step
above.

[tdresser, 2016/04/01 19:03:20]

Let's move the notification up.

+
   if (uncanceledMoveCausesScroll)
-    eventSender.touchMoveCausingScrollIfUncanceled();
-  else
-    eventSender.touchMove();
+    eventSender.notifyStartOfTouchScroll();
 
   eventSender.updateTouchPoint(0, x, y+150);
   eventSender.touchMove();
@@ -98,10 +98,10 @@ function testMultiDrag(uncanceledMoveCausesScroll) {
   eventSender.updateTouchPoint(0, x0, y);
   eventSender.updateTouchPoint(1, x1, y);
   eventSender.updateTouchPoint(2, x2, y);
+  eventSender.touchMove();
+
   if (uncanceledMoveCausesScroll)
-    eventSender.touchMoveCausingScrollIfUncanceled();
-  else
-    eventSender.touchMove();
+    eventSender.notifyStartOfTouchScroll();
 
   eventSender.addTouchPoint(x3, y);
   eventSender.touchStart();
@@ -161,6 +161,8 @@ function runTests() {
   testSingleDrag(false);
   debug("");
 
+  // TODO(mustaq): Scrolling is now independent of whether the TEs thru the eventSender gets
+  // preventDefaulted or not. Nuke preventDefault on touchmove, rename |uncanceledMoveCausesScroll|.
   debug("-- preventDefault on touchmove, without uncanceledMoveCausesScroll --");
   eventToPreventDefault = "touchmove";
   testSingleDrag(false);