Add core plumbing for Instant Extended work

chrome/browser/ui/search/search_model_observer.h

+// Copyright (c) 2012 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 CHROME_BROWSER_UI_SEARCH_SEARCH_MODEL_OBSERVER_H_
+#define CHROME_BROWSER_UI_SEARCH_SEARCH_MODEL_OBSERVER_H_
+#pragma once
+
+namespace chrome {
+namespace search {
+
+struct Mode;
+
+// This class defines the observer interface for the |SearchModel|.
+class SearchModelObserver {
+ public:
+  // Informs the observer that the mode has changed.
+  virtual void ModeChanged(const Mode& mode) = 0;
+
+ protected:
+  virtual ~SearchModelObserver() {}
+};
+
+}  // namespace search
+}  // namespace chrome
+
+#endif  // CHROME_BROWSER_UI_SEARCH_SEARCH_MODEL_OBSERVER_H_