Fix white flash on certain Chromecast screen transitions.

Reboot and brain freeze screens can lead be followed by seeing the
browser compositor's background colour, which defaults to white.

BUG=internal b/19379351

Review URL: https://codereview.chromium.org/950123002

Cr-Commit-Position: refs/heads/master@{#317705}

chromecast/browser/cast_content_window.cc

@@ -85,6 +85,7 @@ void CastContentWindow::CreateWindowTree(
   window_tree_host_->InitHost();
   window_tree_host_->window()->SetLayoutManager(
       new CastFillLayout(window_tree_host_->window()));
+  window_tree_host_->compositor()->SetBackgroundColor(SK_ColorBLACK);
 
   focus_client_.reset(new aura::test::TestFocusClient());
   aura::client::SetFocusClient(