Move the notion of current Tab from TabModel to WebStateList.

ios/chrome/browser/snapshots/snapshot_cache_web_state_list_observer.mm

Index: ios/chrome/browser/snapshots/snapshot_cache_web_state_list_observer.mm
diff --git a/ios/chrome/browser/snapshots/snapshot_cache_web_state_list_observer.mm b/ios/chrome/browser/snapshots/snapshot_cache_web_state_list_observer.mm
new file mode 100644
index 0000000000000000000000000000000000000000..c1031e3af4616a05b44e1da50600d4d0252b67df
--- /dev/null
+++ b/ios/chrome/browser/snapshots/snapshot_cache_web_state_list_observer.mm
@@ -0,0 +1,47 @@
+// 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/snapshots/snapshot_cache_web_state_list_observer.h"
+
+#include "base/logging.h"
+#import "ios/chrome/browser/snapshots/snapshot_cache.h"
+#import "ios/chrome/browser/tabs/legacy_tab_helper.h"
+#import "ios/chrome/browser/tabs/tab.h"
+#import "ios/shared/chrome/browser/tabs/web_state_list.h"
+
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
+SnapshotCacheWebStateListObserver::SnapshotCacheWebStateListObserver(
+    SnapshotCache* snapshot_cache)
+    : snapshot_cache_(snapshot_cache) {
+  DCHECK(snapshot_cache_);
+}
+
+SnapshotCacheWebStateListObserver::~SnapshotCacheWebStateListObserver() =
+    default;
+
+void SnapshotCacheWebStateListObserver::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;
+
+  NSMutableSet<NSString*>* set = [NSMutableSet set];
+  if (active_index > 0) {
+    web::WebState* web_state = web_state_list->GetWebStateAt(active_index - 1);
+    [set addObject:LegacyTabHelper::GetTabForWebState(web_state).tabId];
+  }
+
+  if (active_index + 1 < web_state_list->count()) {
+    web::WebState* web_state = web_state_list->GetWebStateAt(active_index + 1);
+    [set addObject:LegacyTabHelper::GetTabForWebState(web_state).tabId];
+  }
+
+  snapshot_cache_.pinnedIDs = [set copy];
+}