Make GetRootWindow() return a Window instead of a RootWindow.

ash/display/display_manager_unittest.cc

Index: ash/display/display_manager_unittest.cc
diff --git a/ash/display/display_manager_unittest.cc b/ash/display/display_manager_unittest.cc
index 7e8652e08adad26884ab2d20f5509c1857b63c13..255cafac1b03b1dc096c42782ec35d8953fbfa85 100644
--- a/ash/display/display_manager_unittest.cc
+++ b/ash/display/display_manager_unittest.cc
@@ -386,13 +386,13 @@ TEST_F(DisplayManagerTest, TestDeviceScaleOnlyChange) {
     return;
 
   UpdateDisplay("1000x600");
-  EXPECT_EQ(1,
-            Shell::GetPrimaryRootWindow()->compositor()->device_scale_factor());
+  aura::WindowEventDispatcher* dispatcher =
+      Shell::GetPrimaryRootWindow()->GetDispatcher();
+  EXPECT_EQ(1, dispatcher->compositor()->device_scale_factor());
   EXPECT_EQ("1000x600",
             Shell::GetPrimaryRootWindow()->bounds().size().ToString());
   UpdateDisplay("1000x600*2");
-  EXPECT_EQ(2,
-            Shell::GetPrimaryRootWindow()->compositor()->device_scale_factor());
+  EXPECT_EQ(2, dispatcher->compositor()->device_scale_factor());
   EXPECT_EQ("500x300",
             Shell::GetPrimaryRootWindow()->bounds().size().ToString());
 }
@@ -594,8 +594,8 @@ TEST_F(DisplayManagerTest, MAYBE_TestNativeDisplaysChangedNoInternal) {
   EXPECT_EQ(1U, display_manager()->GetNumDisplays());
   EXPECT_EQ("1,1 100x100",
             GetDisplayInfoForId(10).bounds_in_native().ToString());
-  EXPECT_EQ("100x100",
-            ash::Shell::GetPrimaryRootWindow()->GetHostSize().ToString());
+  EXPECT_EQ("100x100", ash::Shell::GetPrimaryRootWindow()->GetDispatcher()->
+      GetHostSize().ToString());
 }
 
 #if defined(OS_WIN)