Move the notion of current Tab from TabModel to WebStateList.

ios/chrome/browser/snapshots/snapshot_cache_web_state_list_observer.h

Index: ios/chrome/browser/snapshots/snapshot_cache_web_state_list_observer.h
diff --git a/ios/chrome/browser/snapshots/snapshot_cache_web_state_list_observer.h b/ios/chrome/browser/snapshots/snapshot_cache_web_state_list_observer.h
new file mode 100644
index 0000000000000000000000000000000000000000..a6e306eaad4496d5ff0afbd9f530742d1d7fc20b
--- /dev/null
+++ b/ios/chrome/browser/snapshots/snapshot_cache_web_state_list_observer.h
@@ -0,0 +1,32 @@
+// 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_SNAPSHOTS_SNAPSHOT_CACHE_WEB_STATE_LIST_OBSERVER_H_
+#define IOS_CHROME_BROWSER_SNAPSHOTS_SNAPSHOT_CACHE_WEB_STATE_LIST_OBSERVER_H_
+
+#include "base/macros.h"
+#import "ios/shared/chrome/browser/tabs/web_state_list_observer.h"
+
+@class SnapshotCache;
+
+// Updates the SnapshotCache when the active Tab changes.
+class SnapshotCacheWebStateListObserver : public WebStateListObserver {
+ public:
+  explicit SnapshotCacheWebStateListObserver(SnapshotCache* snapshot_cache);
+  ~SnapshotCacheWebStateListObserver() override;
+
+ private:
+  // WebStateListObserver implementation.
+  void WebStateActivatedAt(WebStateList* web_state_list,
+                           web::WebState* old_web_state,
+                           web::WebState* new_web_state,
+                           int active_index,
+                           bool user_action) override;
+
+  SnapshotCache* snapshot_cache_;
+
+  DISALLOW_COPY_AND_ASSIGN(SnapshotCacheWebStateListObserver);
+};
+
+#endif  // IOS_CHROME_BROWSER_SNAPSHOTS_SNAPSHOT_CACHE_WEB_STATE_LIST_OBSERVER_H_