ash: Do a null-check for the screen-position client.

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 dde314f09fc4a3ad35a9fa2dae876fcaf7e6603b..e32ddcdc1be537da27923d811a764fc185a271be 100644
--- a/ash/wm/coordinate_conversion.cc
+++ b/ash/wm/coordinate_conversion.cc
@@ -32,9 +32,10 @@ aura::Window* GetRootWindowMatching(const gfx::Rect& rect) {
 }
 
 void ConvertPointToScreen(const aura::Window* window, gfx::Point* point) {
-  CHECK(window);
-  CHECK(window->GetRootWindow());
-  CHECK(aura::client::GetScreenPositionClient(window->GetRootWindow()));
+  // 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;
   aura::client::GetScreenPositionClient(window->GetRootWindow())->
       ConvertPointToScreen(window, point);
 }