Fix window/display bounds with multiple windows in mus.

services/ui/display/platform_screen.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SERVICES_UI_DISPLAY_PLATFORM_SCREEN_H_
 #define SERVICES_UI_DISPLAY_PLATFORM_SCREEN_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 18 matching lines @@
   // Creates a PlatformScreen instance.
   static std::unique_ptr<PlatformScreen> Create();
 
   // Initializes platform specific screen resources.
   virtual void Init() = 0;
 
   // ConfigurePhysicalDisplay() configures a single physical display and returns
   // its id and bounds for it via |callback|.
   virtual void ConfigurePhysicalDisplay(
       const ConfiguredDisplayCallback& callback) = 0;
+
+  virtual int64_t GetPrimaryDisplayId() const = 0;
 };
 
 }  // namespace display
 
 #endif  // SERVICES_UI_DISPLAY_PLATFORM_SCREEN_H_