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 fda4affb3038f172432e0f45dd3c2641d69a66f7..e74d475111215d2db654a8e6f52d234ff0947571 100644
--- a/views/events/event_x.cc
+++ b/views/events/event_x.cc
@@ -122,18 +122,21 @@ ui::EventType GetTouchEventType(XEvent* xev) {
 }
 
 int GetTouchIDFromXEvent(XEvent* xev) {
-  float id = 0;
+  float slot = 0;
+  TouchFactory* factory = 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.
-  TouchFactory::TouchParam tp = TouchFactory::TP_TRACKING_ID;
+  float tracking_id;
+  if (!factory->ExtractTouchParam(
+         *xev, 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
-  TouchFactory::TouchParam tp = TouchFactory::TP_SLOT_ID;
+  if (!factory->ExtractTouchParam(
+         *xev, TouchFactory::TP_SLOT_ID, &slot))
+    LOG(ERROR) << "Could not get the slot ID for the event. Using 0.";
 #endif
-  if (!TouchFactory::GetInstance()->ExtractTouchParam(
-          *xev, tp, &id))
-    LOG(ERROR) << "Could not get the touch ID for the event. Using 0.";
-  return id;
+  return slot;
 }
 
 ui::EventType EventTypeFromNative(NativeEvent2 native_event) {
@@ -424,7 +427,20 @@ TouchEvent::TouchEvent(NativeEvent2 native_event_2,
                                               TouchFactory::TP_ORIENTATION,
                                               0.0)),
       force_(GetTouchForceFromXEvent(native_event_2)) {
-#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.
+    TouchFactory* factory = TouchFactory::GetInstance();
+    float tracking_id;
+    if (factory->ExtractTouchParam(*native_event_2,
+                                   TouchFactory::TP_TRACKING_ID,
+                                   &tracking_id)) {
+      factory->ReleaseSlotForTrackingID(tracking_id);
+    }
+  }
+#else
   if (type() == ui::ET_TOUCH_PRESSED || type() == ui::ET_TOUCH_RELEASED) {
     TouchFactory* factory = TouchFactory::GetInstance();
     float slot;