Move the notion of current Tab from TabModel to WebStateList.

ios/shared/chrome/browser/tabs/web_state_list_metrics_observer.mm

Index: ios/shared/chrome/browser/tabs/web_state_list_metrics_observer.mm
diff --git a/ios/shared/chrome/browser/tabs/web_state_list_metrics_observer.mm b/ios/shared/chrome/browser/tabs/web_state_list_metrics_observer.mm
index e2c201d05ce76ac2deea29de14a4978605951bf4..7d7ec8bf7847e7dbd7eedc5849e32608fd7024d2 100644
--- a/ios/shared/chrome/browser/tabs/web_state_list_metrics_observer.mm
+++ b/ios/shared/chrome/browser/tabs/web_state_list_metrics_observer.mm
@@ -4,6 +4,7 @@
 
 #import "ios/shared/chrome/browser/tabs/web_state_list_metrics_observer.h"
 
+#include "base/metrics/histogram_macros.h"
 #include "base/metrics/user_metrics.h"
 #include "base/metrics/user_metrics_action.h"
 
@@ -11,15 +12,33 @@
 #error "This file requires ARC support."
 #endif
 
-WebStateListMetricsObserver::WebStateListMetricsObserver() = default;
+WebStateListMetricsObserver::WebStateListMetricsObserver() {
+  ResetSessionMetrics();
+}
 
 WebStateListMetricsObserver::~WebStateListMetricsObserver() = default;
 
+void WebStateListMetricsObserver::ResetSessionMetrics() {
+  inserted_web_state_counter_ = 0;
+  detached_web_state_counter_ = 0;
+  activated_web_state_counter_ = 0;
+}
+
+void WebStateListMetricsObserver::RecordSessionMetrics() {
+  UMA_HISTOGRAM_CUSTOM_COUNTS("Session.ClosedTabCounts",
+                              detached_web_state_counter_, 1, 200, 50);
+  UMA_HISTOGRAM_CUSTOM_COUNTS("Session.OpenedTabCounts",
+                              activated_web_state_counter_, 1, 200, 50);
+  UMA_HISTOGRAM_CUSTOM_COUNTS("Session.NewTabCounts",
+                              inserted_web_state_counter_, 1, 200, 50);
+}
+
 void WebStateListMetricsObserver::WebStateInsertedAt(
     WebStateList* web_state_list,
     web::WebState* web_state,
     int index) {
   base::RecordAction(base::UserMetricsAction("MobileNewTabOpened"));
+  ++inserted_web_state_counter_;
 }
 
 void WebStateListMetricsObserver::WebStateReplacedAt(
@@ -37,4 +56,18 @@ void WebStateListMetricsObserver::WebStateDetachedAt(
     web::WebState* web_state,
     int index) {
   base::RecordAction(base::UserMetricsAction("MobileTabClosed"));
+  ++detached_web_state_counter_;
+}
+
+void WebStateListMetricsObserver::WebStateActivatedAt(
+    WebStateList* web_state_list,
+    web::WebState* old_web_state,
+    web::WebState* new_web_state,
+    int active_index,
+    bool user_action) {
+  ++activated_web_state_counter_;
+  if (!user_action)
+    return;
+
+  base::RecordAction(base::UserMetricsAction("MobileTabSwitched"));
 }