[Picture in Picture] Fix ChromeOS crash

When we call UpdateLayerBounds in OverlaySurfaceEmbedder we should
also update the pointer to |window_background_layer_| (like we do
for |video_layer_|).

BUG=863570

Change-Id: Id5b29db2c060705ce0df0cf44cd09fa43da9902f
Reviewed-on: https://chromium-review.googlesource.com/1137062Reviewed-by: apacible <apacible@chromium.org>
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575328}

content/browser/picture_in_picture/overlay_surface_embedder.cc

@@ -51,6 +51,7 @@ void OverlaySurfaceEmbedder::SetPrimarySurfaceId(
 
 void OverlaySurfaceEmbedder::UpdateLayerBounds() {
   // Update the size of window background.
+  window_background_layer_ = window_->GetWindowBackgroundLayer();
   window_background_layer_->SetBounds(
       gfx::Rect(gfx::Point(0, 0), window_->GetBounds().size()));