Remove dispatcher when shutting down RootWindowController

ash/wm/coordinate_conversion.cc

Index: ash/wm/coordinate_conversion.cc
diff --git a/ash/wm/coordinate_conversion.cc b/ash/wm/coordinate_conversion.cc
index e32ddcdc1be537da27923d811a764fc185a271be..991e7d4e1a0d4f4a73b01fdcc155de8ee9186992 100644
--- a/ash/wm/coordinate_conversion.cc
+++ b/ash/wm/coordinate_conversion.cc
@@ -32,10 +32,7 @@ aura::Window* GetRootWindowMatching(const gfx::Rect& rect) {
 }
 
 void ConvertPointToScreen(const aura::Window* window, gfx::Point* point) {
-  // It is possible for the root window to not have a screen position client
-  // when switching multi-monitor mode from extended to mirror.
-  if (!aura::client::GetScreenPositionClient(window->GetRootWindow()))
-    return;
+  CHECK(aura::client::GetScreenPositionClient(window->GetRootWindow()));
   aura::client::GetScreenPositionClient(window->GetRootWindow())->
       ConvertPointToScreen(window, point);
 }