Add support for constrained windows on os x, based on Avi's GTMWindowSheetController.

chrome/browser/tab_contents/constrained_window.h

Index: chrome/browser/tab_contents/constrained_window.h
diff --git a/chrome/browser/tab_contents/constrained_window.h b/chrome/browser/tab_contents/constrained_window.h
index 437f820bc99e670a5a919bca7eededef5aaf8626..a1b8c2abcb84cb3b3862e3b2ea28d1d3fdd5637d 100644
--- a/chrome/browser/tab_contents/constrained_window.h
+++ b/chrome/browser/tab_contents/constrained_window.h
@@ -19,9 +19,8 @@ typedef views::WindowDelegate ConstrainedWindowDelegate;
 class ConstrainedWindowGtkDelegate;
 typedef ConstrainedWindowGtkDelegate ConstrainedWindowDelegate;
 #elif defined(OS_MACOSX)
-// TODO(port): Change this type when Mac gets ConstrainedWindows. It is here
-// only to make things compile. There is no user or consumer of this type.
-typedef void* ConstrainedWindowDelegate;
+class ConstrainedWindowMacDelegate;
+typedef ConstrainedWindowMacDelegate ConstrainedWindowDelegate;
 #endif
 
 class TabContents;