Proper MouseEvent targeting. Adds a Window method that locates a Window for a given point.

aura/desktop_host.h

Index: aura/desktop_host.h
===================================================================
--- aura/desktop_host.h	(revision 98650)
+++ aura/desktop_host.h	(working copy)
@@ -37,8 +37,9 @@
   // Shows the DesktopHost.
   virtual void Show() = 0;
 
-  // Returns the size of the DesktopHost.
+  // Gets/Sets the size of the DesktopHost.
   virtual gfx::Size GetSize() = 0;
+  virtual void SetSize(const gfx::Size& size) = 0;
 };
 
 }  // namespace aura