Move the notion of current Tab from TabModel to WebStateList.

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

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #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"
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #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(
     WebStateList* web_state_list,
     web::WebState* old_web_state,
     web::WebState* new_web_state,
     int index) {
   // Record a tab clobber, since swapping tabs bypasses the Tab code that would
   // normally log clobbers.
   base::RecordAction(base::UserMetricsAction("MobileTabClobbered"));
 }
 
 void WebStateListMetricsObserver::WebStateDetachedAt(
     WebStateList* web_state_list,
     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"));
+}