[Fuchsia] Skip UpdateViewHolderScene until there are overlays

This CL skips these calls until there are overlays present to
avoid marking the primary plane as white in startup.

Bug: 1155736
Change-Id: I4d9f6fce135cf11b70505e045d481531e874834d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2575261Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Commit-Queue: Emircan Uysaler <emircan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834451}

ui/ozone/platform/scenic/scenic_surface.cc

@@ -183,6 +183,8 @@ mojo::PlatformHandle ScenicSurface::CreateView() {
 
 void ScenicSurface::UpdateViewHolderScene() {
   DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);
+  if (overlays_.empty())
+    return;
 
   // |plane_z_order| for main surface is 0.
   int min_z_order = 0;