arc: Prevent crash on ARC++ opt-out

TEST=Locally, no crash reproduced.
BUG=b:142926935
BUG=1016851

Change-Id: I518ec9ed5d03098284a55d2100425fe22987ed23
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1874188Reviewed-by: Mitsuru Oshima (slow) <oshima@chromium.org>
Commit-Queue: Yury Khmel <khmel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708371}

components/exo/client_controlled_shell_surface.cc

@@ -1074,7 +1074,8 @@ void ClientControlledShellSurface::OnPostWidgetCommit() {
 }
 
 void ClientControlledShellSurface::OnSurfaceDestroying(Surface* surface) {
-  client_controlled_state_->ResetDelegate();
+  if (client_controlled_state_)
+    client_controlled_state_->ResetDelegate();
   ShellSurfaceBase::OnSurfaceDestroying(surface);
 }