Move the notion of current Tab from TabModel to WebStateList.

ios/chrome/browser/metrics/tab_usage_recorder_web_state_list_observer.mm

Index: ios/chrome/browser/metrics/tab_usage_recorder_web_state_list_observer.mm
diff --git a/ios/chrome/browser/metrics/tab_usage_recorder_web_state_list_observer.mm b/ios/chrome/browser/metrics/tab_usage_recorder_web_state_list_observer.mm
new file mode 100644
index 0000000000000000000000000000000000000000..f40ea055ab5a2dfc5e1641a89b9fa2922c3b5712
--- /dev/null
+++ b/ios/chrome/browser/metrics/tab_usage_recorder_web_state_list_observer.mm
@@ -0,0 +1,36 @@
+// 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/metrics/tab_usage_recorder_web_state_list_observer.h"
+
+#include "base/logging.h"
+#include "ios/chrome/browser/metrics/tab_usage_recorder.h"
+#include "ios/chrome/browser/tabs/legacy_tab_helper.h"
+
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
+TabUsageRecorderWebStateListObserver::TabUsageRecorderWebStateListObserver(
+    TabUsageRecorder* tab_usage_recorder)
+    : tab_usage_recorder_(tab_usage_recorder) {
+  DCHECK(tab_usage_recorder_);
+}
+
+TabUsageRecorderWebStateListObserver::~TabUsageRecorderWebStateListObserver() =
+    default;
+
+void TabUsageRecorderWebStateListObserver::WebStateActivatedAt(
+    WebStateList* web_state_list,
+    web::WebState* old_web_state,
+    web::WebState* new_web_state,
+    int active_index,
+    bool user_action) {
+  if (!user_action)
+    return;
+
+  tab_usage_recorder_->RecordTabSwitched(
+      old_web_state ? LegacyTabHelper::GetTabForWebState(old_web_state) : nil,
+      new_web_state ? LegacyTabHelper::GetTabForWebState(new_web_state) : nil);
+}