[Chromecast] Add a cast window manager id for settings pane.

Bug: NONE
Test: NONE
Change-Id: I30d29d8d03c5c04e46b62caa48659fc3a5749f40
Reviewed-on: https://chromium-review.googlesource.com/1013288
Commit-Queue: Yilong Yao <yilongyao@google.com>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551052}

chromecast/graphics/cast_window_manager.h

@@ -33,6 +33,7 @@ class CastWindowManager {
     SOFT_KEYBOARD,
     VOLUME,
     MEDIA_INFO,
+    SETTINGS,
     TOP = MEDIA_INFO
   };