exo: Fix multi-display cursor crash

The cursor surface is parented to the primary root window, which may
change when using multiple displays. This CL fixes a crash caused by
removing the child from the primary root window when the parent is a
different root window or the child is orphaned.

BUG=714832
TEST=No crashes after changing primary display.

Review-Url: https://codereview.chromium.org/2862953002
Cr-Commit-Position: refs/heads/master@{#469543}

components/exo/pointer.cc

@@ -89,9 +89,8 @@ void Pointer::SetCursor(Surface* surface, const gfx::Point& hotspot) {
     }
     if (surface_) {
       surface_->window()->SetTransform(gfx::Transform());
-      WMHelper::GetInstance()
-          ->GetContainer(ash::kShellWindowId_MouseCursorContainer)
-          ->RemoveChild(surface_->window());
+      if (surface_->window()->parent())
+        surface_->window()->parent()->RemoveChild(surface_->window());
       surface_->SetSurfaceDelegate(nullptr);
       surface_->RemoveSurfaceObserver(this);
     }