[Mac]Implement ViewID support.

chrome/browser/cocoa/view_id_util.h

Index: chrome/browser/cocoa/view_id_util.h
diff --git a/chrome/browser/cocoa/view_id_util.h b/chrome/browser/cocoa/view_id_util.h
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+// Copyright (c) 2010 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_COCOA_VIEW_ID_UTIL_H_
+#define CHROME_BROWSER_COCOA_VIEW_ID_UTIL_H_
+
+#include "gfx/native_widget_types.h"
+#include "chrome/browser/view_ids.h"
+
+namespace view_id_util {
+
+// We use NSView's tag property to support ViewID, so we don't need special code
+// for binding a ViewID to a NSView. The subclasses of NSView shall override tag
+// method to return their ViewIDs.
+// However the tag property is also used elsewhere, such as the window buttons
+// provided by the Cocoa system (the three stoplights on the top left corner),
+// whose tags will be set to 0, 1, 2 by the Cocoa system.
+// In order to avoid conflict, we use negative tag value for ViewID.
+// No existing chromium code uses negative tag value, so it's safe for us to use
+// it.
+// See http://dev.chromium.org/developers/design-documents/viewid-support-on-mac
+// for details.
+
+// A convenient function to convert a ViewID value to negative tag value. Using
+// this function to make sure that we only need to change this fuction when we
+// have to change the mapping algorithm to avoid conflict in the future.
+inline int ViewIDToTag(ViewID viewID) {
+  return -viewID - 1;
+}
+
+// A convenient function to lookup a view by a ViewID in a given window.
+NSView* GetView(NSWindow* window, ViewID viewID);
+
+}  // namespace view_id_util
+
+#endif  // CHROME_BROWSER_COCOA_VIEW_ID_UTIL_H_