Fix window/display bounds with multiple windows in mus.

services/ui/display/platform_screen_impl.cc

Index: services/ui/display/platform_screen_impl.cc
diff --git a/services/ui/display/platform_screen_impl.cc b/services/ui/display/platform_screen_impl.cc
index 960c4caab34f0d2a8fd8851f8b822e524d48badd..702906fbbc246d7161564e61462c7dce08e1f953 100644
--- a/services/ui/display/platform_screen_impl.cc
+++ b/services/ui/display/platform_screen_impl.cc
@@ -13,9 +13,11 @@
 namespace display {
 namespace {
 
+const int64_t kDisplayId = 1;
+
 void FixedSizeScreenConfiguration(
     const PlatformScreen::ConfiguredDisplayCallback& callback) {
-  callback.Run(1, gfx::Rect(1024, 768));
+  callback.Run(kDisplayId, gfx::Rect(1024, 768));
 }
 
 }  // namespace
@@ -37,4 +39,8 @@ void PlatformScreenImpl::ConfigurePhysicalDisplay(
       FROM_HERE, base::Bind(&FixedSizeScreenConfiguration, callback));
 }
 
+int64_t PlatformScreenImpl::GetPrimaryDisplayId() const {
+  return kDisplayId;
+}
+
 }  // namespace display