Adding search widget UI and functionality.

ios/chrome/search_widget_extension/ui_util.h

+// 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_SEARCH_WIDGET_EXTENSION_UI_UTIL_H_
+#define IOS_CHROME_SEARCH_WIDGET_EXTENSION_UI_UTIL_H_
+
+namespace ui_util {
+
+// Returns constraints to make two views' size and center equal by pinning
+// leading, trailing, top and bottom anchors.
+NSArray<NSLayoutConstraint*>* CreateSameConstraints(UIView* view1,
+                                                    UIView* view2);
+
+}  // namespace ui_util
+
+#endif  // IOS_CHROME_SEARCH_WIDGET_EXTENSION_UI_UTIL_H_