[ios] Take snapshot for tab grid.

ios/chrome/browser/snapshots/snapshot_constants.mm

Index: ios/chrome/browser/snapshots/snapshot_constants.mm
diff --git a/ios/chrome/browser/snapshots/snapshot_constants.mm b/ios/chrome/browser/snapshots/snapshot_constants.mm
new file mode 100644
index 0000000000000000000000000000000000000000..34e2925916f886615ab8160c1eb82b12bf571b82
--- /dev/null
+++ b/ios/chrome/browser/snapshots/snapshot_constants.mm
@@ -0,0 +1,11 @@
+// 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_constants.h"
+
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
+const CGSize kSnapshotThumbnailSize = {250.0f, 250.0f};