chromeos: fix ShellPortMash::Get() reference

ShellPortMash is no longer always created. The code path this code
calls is only applicable to mash, so can early out for mus.

BUG=none
TEST=none

Change-Id: I55c58c353c0ebeba70cfec313a62c845e00faee9
Reviewed-on: https://chromium-review.googlesource.com/747204Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512919}

ash/mus/window_manager.cc

@@ -486,6 +486,8 @@ ui::mojom::EventResult WindowManager::OnAccelerator(
 }
 
 void WindowManager::OnCursorTouchVisibleChanged(bool enabled) {
+  // Not applicable to Config::MUS.
+  if (config_ == Config::MASH)
     ShellPortMash::Get()->OnCursorTouchVisibleChanged(enabled);
 }