Display: dectect changes to mirror mode and send a notification.

ui/display/manager/display_manager.h

Index: ui/display/manager/display_manager.h
diff --git a/ui/display/manager/display_manager.h b/ui/display/manager/display_manager.h
index e69453ac93d0a59e6a682220969fe29e1a3728c2..5524f3183fd9118373edb52df80a9f6105cf381d 100644
--- a/ui/display/manager/display_manager.h
+++ b/ui/display/manager/display_manager.h
@@ -471,6 +471,9 @@ class DISPLAY_MANAGER_EXPORT DisplayManager
   int64_t mirroring_display_id_ = kInvalidDisplayId;
   Displays software_mirroring_display_list_;
 
+  // Cached mirror mode for metrics changed notification.
+  bool mirror_mode_for_metrics_ = false;

[stevenjb, 2017/04/01 00:38:02]

I tried to avoid the need for this, but the code paths for setting
mirroring_display_id_ (which is how we identify mirror mode) is.... complicated,
and I was nervous about changing it.

[oshima, 2017/04/03 19:38:17]

Can't you check before and after like

boolean wasInMirror = IsInMirrorMode();

update goes on


boolean isInMirror = IsInMirrorMode();

if (wasInMirro != isInMirror) {
}
?

[stevenjb, 2017/04/03 19:49:50]

Nope. mirroring_display_id_ (which is what IsInMirrorMode() checks) gets set
before UpdateDisplaysWith() is called. UpdateDisplaysWith() gets called from 10
call sites, and mirroring_display_id_ gets set in 5 different places, so
tracking down where/when/why it gets set is beyond me. I'd be happy to review a
change that cleans all that up though :)

+
   // User preference for rotation lock of the internal display.
   bool registered_internal_display_rotation_lock_ = false;