Convert some of TabModel logic to WebStateListObservers.

ios/chrome/browser/tabs/tab_model_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/chrome/browser/tabs/tab_model_metrics_observer.h"

[marq (ping after 24h), 2017/02/13 17:06:54]

Can you put this in ios/shared/, since it's free of dependencies, and we should
probably just reuse it?

[sdefresne, 2017/02/14 17:19:38]

Should I rename it WebStateListMetricsObserver then?

+
+#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
+
+@implementation TabModelMetricsObserver
+
+#pragma mark WebStateListObserver
+
+- (void)webStateList:(WebStateList*)webStateList
+    webStateInserted:(web::WebState*)webState
+             atIndex:(NSUInteger)index {
+  base::RecordAction(base::UserMetricsAction("MobileNewTabOpened"));
+}
+
+- (void)webStateList:(WebStateList*)webStateList
+    webStateReplaced:(web::WebState*)oldWebState
+          byWebState:(web::WebState*)newWebState
+             atIndex:(NSUInteger)index {
+  // Record a tab clobber, since swapping tabs bypasses the Tab code that would
+  // normally log clobbers.
+  base::RecordAction(base::UserMetricsAction("MobileTabClobbered"));
+}
+
+- (void)webStateList:(WebStateList*)webStateList
+     webStateRemoved:(web::WebState*)webState
+             atIndex:(NSUInteger)index {
+  base::RecordAction(base::UserMetricsAction("MobileTabClosed"));
+}
+
+@end