chromeos: fix ifdefs

Apparently there is still an x11 chromeos bot. This should fix the
build for it.

TBR as this is trivial change to add ifdefs and should fix bot.

BUG=701068
TEST=none
TBR=erg@chromium.org

Change-Id: I55aad4f264ece847338e98eac635262c1583ac26
Reviewed-on: https://chromium-review.googlesource.com/576280Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Elliot Glaysher <erg@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487529}

ash/mus/ash_window_tree_host_mus.cc

@@ -74,6 +74,7 @@ void AshWindowTreeHostMus::RegisterMirroringHost(
   NOTIMPLEMENTED();
 }
 
+#if defined(USE_OZONE)
 void AshWindowTreeHostMus::SetCursorConfig(
     const display::Display& display,
     display::Display::Rotation rotation) {
@@ -83,6 +84,7 @@ void AshWindowTreeHostMus::SetCursorConfig(
 void AshWindowTreeHostMus::ClearCursorConfig() {
   // Nothing to do here, mus takes care of this.
 }
+#endif
 
 void AshWindowTreeHostMus::SetRootTransform(const gfx::Transform& transform) {
   transformer_helper_->SetTransform(transform);