touchui: Convert XI2 MT tracking id to slot id for gesture recognizer

views/events/event_x.cc

Index: views/events/event_x.cc
diff --git a/views/events/event_x.cc b/views/events/event_x.cc
index 9957d5f00941bea07e04e6147881e9bd13df8625..86995fe7bf03462fee134e194f031fd5e0903e79 100644
--- a/views/events/event_x.cc
+++ b/views/events/event_x.cc
@@ -20,17 +20,21 @@ namespace views {
 namespace {
 
 int GetTouchIDFromXEvent(XEvent* xev) {
-  float id = 0;
+  float slot = 0;
+  ui::TouchFactory* factory = ui::TouchFactory::GetInstance();
 #if defined(USE_XI2_MT)
-  // TODO(ningxin.hu@gmail.com): Make the id always start from 0 for a new
-  // touch-sequence when TRACKING_ID is used to extract the touch id.
-  ui::TouchFactory::TouchParam tp = ui::TouchFactory::TP_TRACKING_ID;
+  float tracking_id;
+  if (!factory->ExtractTouchParam(
+         *xev, ui::TouchFactory::TP_TRACKING_ID, &tracking_id))
+    LOG(ERROR) << "Could not get the slot ID for the event. Using 0.";
+  else
+    slot = factory->GetSlotForTrackingID(tracking_id);
 #else
-  ui::TouchFactory::TouchParam tp = ui::TouchFactory::TP_SLOT_ID;
+  if (!factory->ExtractTouchParam(
+         *xev, ui::TouchFactory::TP_SLOT_ID, &slot))
+    LOG(ERROR) << "Could not get the slot ID for the event. Using 0.";
 #endif
-  if (!ui::TouchFactory::GetInstance()->ExtractTouchParam(*xev, tp, &id))
-    LOG(ERROR) << "Could not get the touch ID for the event. Using 0.";
-  return id;
+  return slot;
 }
 
 uint16 GetCharacterFromXKeyEvent(XKeyEvent* key) {
@@ -171,7 +175,20 @@ TouchEvent::TouchEvent(const ui::NativeEvent& native_event)
                                               ui::TouchFactory::TP_ORIENTATION,
                                               0.0)),
       force_(GetTouchForceFromXEvent(native_event)) {
-#if !defined(USE_XI2_MT)
+#if defined(USE_XI2_MT)
+  if (type() == ui::ET_TOUCH_RELEASED) {
+    // NOTE: The slot is allocated by TouchFactory for each XI_TouchBegin
+    //       event, which carries a new tracking ID to identify a new touch
+    //       sequence.
+    ui::TouchFactory* factory = ui::TouchFactory::GetInstance();
+    float tracking_id;
+    if (factory->ExtractTouchParam(*native_event,
+                                   ui::TouchFactory::TP_TRACKING_ID,
+                                   &tracking_id)) {
+      factory->ReleaseSlotForTrackingID(tracking_id);
+    }
+  }
+#else
   if (type() == ui::ET_TOUCH_PRESSED || type() == ui::ET_TOUCH_RELEASED) {
     ui::TouchFactory* factory = ui::TouchFactory::GetInstance();
     float slot;