Add GetDisplayWithDisplayId to display::Screen.

ash/test/ash_test_base.h

Index: ash/test/ash_test_base.h
diff --git a/ash/test/ash_test_base.h b/ash/test/ash_test_base.h
index c93451ee337b2493f74d4a1f37299f873fcc7df8..e8c4b63d46821980e232332553f15a2ee1a33f08 100644
--- a/ash/test/ash_test_base.h
+++ b/ash/test/ash_test_base.h
@@ -77,6 +77,12 @@ class AshTestBase : public testing::Test {
   // See ash::test::DisplayManagerTestApi::UpdateDisplay for more details.
   void UpdateDisplay(const std::string& display_specs);
 
+  // Set the 1st display as an internal display and returns the display Id for
+  // the internal display.
+  // See ash::test::DisplayManagerTestApi::SetFirstDisplayAsInternalDisplay for
+  // more details.
+  int64_t SetFirstDisplayAsInternalDisplay();
+
   // Returns a root Window. Usually this is the active root Window, but that
   // method can return NULL sometimes, and in those cases, we fall back on the
   // primary root Window.