Introdcue AshWindowTreeHost and move ash/chrome specific code in WTH to ash.

ash/test/ash_test_helper.cc

Index: ash/test/ash_test_helper.cc
diff --git a/ash/test/ash_test_helper.cc b/ash/test/ash_test_helper.cc
index f66c6ce3907635d48efa4624f8a965057f408a22..5a0f438c7cc4f57b8bce05f743f72e214e88a1e6 100644
--- a/ash/test/ash_test_helper.cc
+++ b/ash/test/ash_test_helper.cc
@@ -34,6 +34,10 @@
 #include "ui/aura/window_tree_host_x11.h"
 #endif
 
+#if defined(OS_WIN)
+#include "ash/host/ash_remote_window_tree_host_win.h"
+#endif
+
 namespace ash {
 namespace test {
 
@@ -52,6 +56,9 @@ AshTestHelper::~AshTestHelper() {
 }
 
 void AshTestHelper::SetUp(bool start_session) {
+#if defined(OS_WIN)
+  AshRemoteWindowTreeHostWin::Init();
+#endif
   wm_state_.reset(new wm::WMState);
 
   // Disable animations during tests.