Convert some of TabModel logic to WebStateListObservers.

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/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() = default;
+
+void WebStateListMetricsObserver::WebStateInsertedAt(
+    WebStateList* web_state_list,
+    web::WebState* web_state,
+    int index) {
+  base::RecordAction(base::UserMetricsAction("MobileNewTabOpened"));
+}
+
+void WebStateListMetricsObserver::WebStateMoved(WebStateList* web_state_list,
+                                                web::WebState* web_state,
+                                                int from_index,
+                                                int to_index) {}
+
+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"));
+}