viz: Avoid Null Crash for OverlayProcessor in Display

When viz::Display receives swap buffers ack, it checks that |renderer_|
is not null. This implies this function could be called after
|renderer_| is deleted. By the same logic we would need to check for
|overlay_processor_| not being null either.

Bug: 1048041
Change-Id: Ib57ee5151eaae3b885c55f478959f747a96ab837
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2035301Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Commit-Queue: weiliangc <weiliangc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737814}

components/viz/service/display/display.cc

@@ -724,7 +724,8 @@ void Display::DidReceiveSwapBuffersAck(const gfx::SwapTimings& timings) {
   if (no_pending_swaps_callback_ && pending_swaps_ == 0)
     std::move(no_pending_swaps_callback_).Run();
 
-  overlay_processor_->OverlayPresentationComplete();
+  if (overlay_processor_)
+    overlay_processor_->OverlayPresentationComplete();
   if (renderer_)
     renderer_->SwapBuffersComplete();