chromeos: Eliminate WmWindow from //ash/system

ash/system/palette/palette_tool_manager_unittest.cc

Index: ash/system/palette/palette_tool_manager_unittest.cc
diff --git a/ash/system/palette/palette_tool_manager_unittest.cc b/ash/system/palette/palette_tool_manager_unittest.cc
index e8eef463e2a610de18c871273fa7670df07f49c6..cebd8cb18e5792b4e281dd763630542527c11a86 100644
--- a/ash/system/palette/palette_tool_manager_unittest.cc
+++ b/ash/system/palette/palette_tool_manager_unittest.cc
@@ -55,7 +55,7 @@ class PaletteToolManagerTest : public ::testing::Test,
   void HidePalette() override {}
   void HidePaletteImmediately() override {}
   void OnActiveToolChanged() override { ++tool_changed_count_; }
-  WmWindow* GetWindow() override {
+  aura::Window* GetWindow() override {
     NOTREACHED();
     return nullptr;
   }