Create RootWindowTransformer after root window's bounds and scale factor are updated.

ash/test/ash_test_base.h

Index: ash/test/ash_test_base.h
diff --git a/ash/test/ash_test_base.h b/ash/test/ash_test_base.h
index 53c2e50467c5d7d254fb8a3439ae196086ca5d1e..69564b06ae3ddb93988447bbbd98b79acee3cfe2 100644
--- a/ash/test/ash_test_base.h
+++ b/ash/test/ash_test_base.h
@@ -99,6 +99,10 @@ class AshTestBase : public testing::Test {
   // or false otherwise (e.g. win8 bot).
   static bool SupportsMultipleDisplays();
 
+  // True if the running environment supports host window resize,
+  // or false otherwise (e.g. win8 bot).
+  static bool SupportsHostWindowResize();
+
   void RunAllPendingInMessageLoop();
 
   // Utility methods to emulate user logged in or not, session started or not