[Chromecast] Add additional window types

Add a BOOT_ANIMATION_OVERLAY type for the initial animation happening at
startup.

Add a CORNERS_OVERLAY type for window mask around the display corners.

BUG=b/110092844
TEST=None

Change-Id: Ifde30aeb490171e2f3daf0bb727f9e70e6e6ced4
Reviewed-on: https://chromium-review.googlesource.com/1103301
Commit-Queue: Daniel Nicoara <dnicoara@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568107}

chromecast/graphics/cast_window_manager.h

@@ -34,7 +34,9 @@ class CastWindowManager {
     VOLUME,
     MEDIA_INFO,
     SETTINGS,
-    TOP = MEDIA_INFO
+    CORNERS_OVERLAY,
+    BOOT_ANIMATION_OVERLAY,
+    TOP = BOOT_ANIMATION_OVERLAY
   };
 
   virtual ~CastWindowManager() {}