Convert some of TabModel logic to WebStateListObservers.

ios/chrome/browser/tabs/tab_model_metrics_observer.h

Index: ios/chrome/browser/tabs/tab_model_metrics_observer.h
diff --git a/ios/chrome/browser/tabs/tab_model_metrics_observer.h b/ios/chrome/browser/tabs/tab_model_metrics_observer.h
new file mode 100644
index 0000000000000000000000000000000000000000..f3334195bb09d5d7a764d30c90de783354288924
--- /dev/null
+++ b/ios/chrome/browser/tabs/tab_model_metrics_observer.h
@@ -0,0 +1,14 @@
+// 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.
+
+#ifndef IOS_CHROME_BROWSER_TABS_TAB_MODEL_METRICS_OBSERVER_H_
+#define IOS_CHROME_BROWSER_TABS_TAB_MODEL_METRICS_OBSERVER_H_
+
+#import "ios/shared/chrome/browser/tabs/web_state_list_observer_bridge.h"
+
+// Listen to WebStateList for TabModel and update metrics.
+@interface TabModelMetricsObserver : NSObject<WebStateListObserver>

[rohitrao (ping after 24h), 2017/02/14 13:29:05]

Could this be a C++ object?  Is there a benefit to having it in ObjC?

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

Done.

+@end
+
+#endif  // IOS_CHROME_BROWSER_TABS_TAB_MODEL_METRICS_OBSERVER_H_