Remove deprecated OS_WIN checks in src/ash

Ash is no longer built on Windows.
Also cleanup DragDropControllerTest.

Bug: NONE
Change-Id: I73b3ce606aca98abc6248f7cf8356c6307a785b6
Reviewed-on: https://chromium-review.googlesource.com/723866Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509543}

ash/display/cursor_window_controller_unittest.cc

@@ -114,8 +114,6 @@ TEST_F(CursorWindowControllerTest, MoveToDifferentDisplay) {
   EXPECT_EQ(50, cursor_bounds.y() + hot_point.y());
 }
 
-// Windows doesn't support compositor based cursor.
-#if !defined(OS_WIN)
 // Make sure that composition cursor inherits the visibility state.
 TEST_F(CursorWindowControllerTest, VisibilityTest) {
   ASSERT_TRUE(GetCursorWindow());
@@ -169,6 +167,5 @@ TEST_F(CursorWindowControllerTest, DSF) {
       display::Screen::GetScreen()->GetPrimaryDisplay().device_scale_factor());
   EXPECT_TRUE(GetCursorImage().HasRepresentation(2.0f));
 }
-#endif
 
 }  // namespace ash