Add metrics for the number of attached displays.

ui/gfx/screen_mac.mm

Index: ui/gfx/screen_mac.mm
diff --git a/ui/gfx/screen_mac.mm b/ui/gfx/screen_mac.mm
index 2bb90e6738cc9307b53285c0d6e8b02898684c88..8a1b0e08ecef17e30ada9eed686f8638d4490930 100644
--- a/ui/gfx/screen_mac.mm
+++ b/ui/gfx/screen_mac.mm
@@ -91,6 +91,11 @@ class ScreenMac : public gfx::Screen {
         ScreenMac::DisplayReconfigurationCallBack, this);
   }
 
+  virtual ~ScreenMac() {
+    CGDisplayRemoveReconfigurationCallback(
+        ScreenMac::DisplayReconfigurationCallBack, this);
+  }
+
   gfx::Point GetCursorScreenPoint() override {
     NSPoint mouseLocation  = [NSEvent mouseLocation];
     // Flip coordinates to gfx (0,0 in top-left corner) using primary screen.