Move the notion of current Tab from TabModel to WebStateList.

ios/chrome/browser/metrics/BUILD.gn

 # Copyright 2016 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("metrics") {
   configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "field_trial_synchronizer.cc",
     "field_trial_synchronizer.h",
     "ios_chrome_metrics_service_accessor.cc",
@@ skipping 91 matching lines @@
     "//base",
     "//components/google/core/browser",
     "//ios/chrome/browser",
     "//ios/chrome/browser/browser_state",
     "//ios/chrome/browser/tabs",
     "//ios/chrome/browser/ui",
     "//ios/web",
     "//ui/base",
     "//url",
   ]
+  public_deps = [
+    ":metrics_internal_arc",
+  ]
+  allow_circular_includes_from = [ ":metrics_internal_arc" ]
   libs = [ "UIKit.framework" ]
 }
 
+source_set("metrics_internal_arc") {
+  sources = [
+    "tab_usage_recorder_web_state_list_observer.h",
+    "tab_usage_recorder_web_state_list_observer.mm",
+  ]
+  deps = [
+    "//base",
+    "//ios/chrome/browser/tabs",
+    "//ios/shared/chrome/browser/tabs",
+  ]
+  configs += [ "//build/config/compiler:enable_arc" ]
+}
+
 source_set("unit_tests_internal") {
   testonly = true
   sources = [
     "first_user_action_recorder_unittest.cc",
     "size_class_recorder_unittest.mm",
     "tab_usage_recorder_unittest.mm",
   ]
   deps = [
     ":metrics",
     ":metrics_internal",
@@ skipping 29 matching lines @@
     "//ios/chrome/test/app:test_support",
     "//ios/chrome/test/earl_grey:test_support",
     "//ios/testing:ios_test_support",
     "//ios/third_party/earl_grey",
     "//ios/web:earl_grey_test_support",
     "//ios/web:test_support",
     "//ui/base",
   ]
   libs = [ "XCTest.framework" ]
 }