Add TRANSIENTLY_HIDDEN to VisibilityType

This is needed to notify cast apps when they are covered by other
activities.

Bug: b/120809866
Test: Compile
Change-Id: I312117a7b051183ad0056cfeff2d0edff49a6f82
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1900388Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Reviewed-by: Sean Topping <seantopping@chromium.org>
Commit-Queue: Shawn Gallea <sagallea@google.com>
Cr-Commit-Position: refs/heads/master@{#713520}

chromecast/browser/cast_content_window.h

@@ -32,11 +32,16 @@ enum class VisibilityType {
   // Window occupies a portion of the screen, supporting user interaction.
   PARTIAL_OUT = 2,
 
-  // Window is hidden, and cannot be interacted with via touch.
+  // Window is hidden after dismissal by back gesture, and cannot be interacted
+  // with via touch.
   HIDDEN = 3,
 
   // Window is being displayed as a small visible tile.
-  TILE = 4
+  TILE = 4,
+
+  // Window is covered by other activities and cannot be interacted with via
+  // touch.
+  TRANSIENTLY_HIDDEN = 5
 };
 
 // Represents requested activity windowing behavior. Behavior includes: