Convert some of TabModel logic to WebStateListObservers.

ios/shared/chrome/browser/tabs/BUILD.gn

 # 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.
 
 source_set("tabs") {
   sources = [
     "web_state_list.h",
     "web_state_list.mm",
     "web_state_list_fast_enumeration_helper.h",
     "web_state_list_fast_enumeration_helper.mm",
+    "web_state_list_metrics_observer.h",
+    "web_state_list_metrics_observer.mm",
     "web_state_list_observer.h",
     "web_state_list_observer_bridge.h",
     "web_state_list_observer_bridge.mm",
   ]
   deps = [
     "//base",
     "//ios/web",
   ]
   libs = [ "Foundation.framework" ]
   configs += [ "//build/config/compiler:enable_arc" ]
 }
 
 source_set("unit_tests") {
   testonly = true
   sources = [
     "web_state_list_fast_enumeration_helper_unittest.mm",
     "web_state_list_unittest.mm",
   ]
   deps = [
     ":tabs",
     "//base",
     "//ios/web:test_support",
     "//net",
     "//testing/gtest",
     "//url",
   ]
   configs += [ "//build/config/compiler:enable_arc" ]
 }