Add GetDisplayWithDisplayId to display::Screen.

blimp/engine/app/ui/blimp_screen_unittest.cc

Index: blimp/engine/app/ui/blimp_screen_unittest.cc
diff --git a/blimp/engine/app/ui/blimp_screen_unittest.cc b/blimp/engine/app/ui/blimp_screen_unittest.cc
index dd6cc6427db3ee687279a3652fe22549f8ee8ff7..ee4fe61c2087e428c3185f6cfb145ca2f6c3a22b 100644
--- a/blimp/engine/app/ui/blimp_screen_unittest.cc
+++ b/blimp/engine/app/ui/blimp_screen_unittest.cc
@@ -114,6 +114,16 @@ TEST_F(BlimpScreenTest, RemoveObserver) {
   EXPECT_EQ(size1, screen_->GetPrimaryDisplay().GetSizeInPixel());
 }
 
+TEST_F(BlimpScreenTest, GetDisplayById) {
+  // display_ in BlimpScreen has id 1
+  display::Display display;
+  bool has_display1 = screen_->GetDisplayWithDisplayId(0, &display);
+  EXPECT_FALSE(has_display1);
+  bool has_display2 = screen_->GetDisplayWithDisplayId(1, &display);
+  EXPECT_TRUE(has_display2);
+  EXPECT_EQ(1, display.id());
+}
+
 }  // namespace
 }  // namespace engine
 }  // namespace blimp