Make it possible to change the viewport size

mojo/services/view_manager/display_manager.cc

Index: mojo/services/view_manager/display_manager.cc
diff --git a/mojo/services/view_manager/display_manager.cc b/mojo/services/view_manager/display_manager.cc
index fa91e2da67a5745829ef49b42788832697dc5db9..b2d7933c6e1078289a80281d75c8c4d36247ebdd 100644
--- a/mojo/services/view_manager/display_manager.cc
+++ b/mojo/services/view_manager/display_manager.cc
@@ -113,6 +113,10 @@ void DisplayManager::SchedulePaint(const ServerView* view,
   }
 }
 
+void DisplayManager::SetViewportSize(const gfx::Size& size) {
+  native_viewport_->SetSize(Size::From(size));
+}
+
 void DisplayManager::OnCreatedNativeViewport(uint64_t native_viewport_id) {
 }