Unify display ids between Display and PlatformDisplay.

services/ui/ws/display_manager.h

Index: services/ui/ws/display_manager.h
diff --git a/services/ui/ws/display_manager.h b/services/ui/ws/display_manager.h
index 4c7847db26b60e89221618b35e6a14abee4fbeb9..3409f377ee4d77636357c27b250494920159d336 100644
--- a/services/ui/ws/display_manager.h
+++ b/services/ui/ws/display_manager.h
@@ -68,8 +68,6 @@ class DisplayManager : public UserIdTrackerObserver {
   // as well as the root of WindowManagers).
   WindowId GetAndAdvanceNextRootId();
 
-  uint32_t GetAndAdvanceNextDisplayId();
-
   // Called when the AcceleratedWidget is available for |display|.
   void OnDisplayAcceleratedWidgetAvailable(Display* display);
 
@@ -91,8 +89,6 @@ class DisplayManager : public UserIdTrackerObserver {
   // ID to use for next root node.
   ClientSpecificId next_root_id_;
 
-  uint32_t next_display_id_;
-
   DISALLOW_COPY_AND_ASSIGN(DisplayManager);
 };