Land http://codereview.chromium.org/159067:

chrome/common/view_types.h

Index: chrome/common/view_types.h
diff --git a/chrome/common/view_types.h b/chrome/common/view_types.h
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+// Copyright (c) 2006-2009 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_COMMON_VIEW_TYPES_H_
+#define CHROME_COMMON_VIEW_TYPES_H_
+
+#include "base/basictypes.h"
+
+// Indicates different types of views
+class ViewType {
+ public:
+  enum Type {
+    INVALID,
+    TAB_CONTENTS,
+    EXTENSION_TOOLSTRIP,
+    EXTENSION_BACKGROUND_PAGE,
+    DEV_TOOLS_UI,
+    INTERSTITIAL_PAGE,
+  };
+
+ private:
+  // This class is for scoping only, so you shouldn't create an instance of it.
+  ViewType() {}
+
+  DISALLOW_COPY_AND_ASSIGN(ViewType);
+};
+
+#endif  // CHROME_COMMON_VIEW_TYPES_H_