Use persistent ID/names for displays.

ui/aura/display_change_observer_x11.cc

Index: ui/aura/display_change_observer_x11.cc
diff --git a/ui/aura/display_change_observer_x11.cc b/ui/aura/display_change_observer_x11.cc
index 0d863803135f1182ac2db5d6bb75d139af9b12fa..540f76904a8122851a4a9156ba91b081f225c74f 100644
--- a/ui/aura/display_change_observer_x11.cc
+++ b/ui/aura/display_change_observer_x11.cc
@@ -15,6 +15,7 @@
 #include "ui/aura/dispatcher_linux.h"
 #include "ui/aura/env.h"
 #include "ui/aura/display_manager.h"
+#include "ui/base/x/x11_util.h"
 #include "ui/compositor/dip_util.h"
 #include "ui/gfx/display.h"
 
@@ -150,6 +151,18 @@ void DisplayChangeObserverX11::NotifyDisplayChange() {
         kHighDensityDIPThreshold) {
       device_scale_factor = 2.0f;
     }
+
+    uint16 manufacturer_id = 0;
+    uint32 serial_number = 0;
+    std::string unused_name;
+    if (ui::GetOutputDeviceData(screen_resources->outputs[o], &manufacturer_id,
+                                &serial_number, &unused_name)) {
+      // Fallback to use index in case that it fails to retrieve these data.
+      // See below.
+      displays.back().set_id(
+          gfx::Display::GetID(manufacturer_id, serial_number));
+    }
+
     displays.back().set_device_scale_factor(device_scale_factor);
     y_coords.insert(crtc_info->y);
     XRRFreeOutputInfo(output_info);
@@ -165,11 +178,14 @@ void DisplayChangeObserverX11::NotifyDisplayChange() {
   // PowerManager lays out the outputs vertically. Sort them by Y
   // coordinates.
   std::sort(displays.begin(), displays.end(), CompareDisplayY);
-  // TODO(oshima): Assisgn index as ID for now. Use unique ID.
   int id = 0;
   for (std::vector<gfx::Display>::iterator iter = displays.begin();
-       iter != displays.end(); ++iter, ++id)
-    (*iter).set_id(id);
+       iter != displays.end(); ++iter) {
+    if (iter->id() == -1) {
+      iter->set_id(id);
+      ++id;
+    }
+  }
 
   Env::GetInstance()->display_manager()->OnNativeDisplaysChanged(displays);
 }