[Chromecast] Add HIDDEN_STICKY VisibilityPriority

This is necessary for content apps which obscured by an opaque overlay
(such as the remote control overlay), but still want to be treated as
fully visible.

Bug: internal b/113950519
Test: None
Change-Id: Ifc2923e91900845e33a90bb832d05586ea711990
Reviewed-on: https://chromium-review.googlesource.com/1226261Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Commit-Queue: Sean Topping <seantopping@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591202}

chromecast/browser/cast_content_window.h

@@ -63,6 +63,11 @@ enum class VisibilityPriority {
 
   // The activity should not be visible.
   HIDDEN = 5,
+
+  // The activity should not be visible, but the activity will consider itself
+  // to be visible. This is useful for opaque overlays while the activity is
+  // still active.
+  HIDDEN_STICKY = 6,
 };
 
 enum class GestureType {