Plumb gesture source value through Blink.

components/test_runner/event_sender.cc

Index: components/test_runner/event_sender.cc
diff --git a/components/test_runner/event_sender.cc b/components/test_runner/event_sender.cc
index 958fadd651c703b5e14e7d048a3aa54ef39d0ccd..9560ea3393c47a9f3fd54a3636348736a5bc9da2 100644
--- a/components/test_runner/event_sender.cc
+++ b/components/test_runner/event_sender.cc
@@ -1663,6 +1663,10 @@ void EventSender::DumpFilenameBeingDragged() {
 void EventSender::GestureFlingCancel() {
   WebGestureEvent event;
   event.type = WebInputEvent::GestureFlingCancel;
+  // Generally it won't matter what device we use here, and since it might
+  // be cumbersome to expect all callers to specify a device, we'll just
+  // choose Touchpad here.
+  event.sourceDevice = blink::WebGestureDeviceTouchpad;
   event.timeStampSeconds = GetCurrentEventTimeSec();
 
   if (force_layout_on_events_)