Adds support for multiple displays to WmTestBase

ash/mus/test/wm_test_base.h

Index: ash/mus/test/wm_test_base.h
diff --git a/ash/mus/test/wm_test_base.h b/ash/mus/test/wm_test_base.h
index 8872208ccdfa4de49aaf567851754569a071dd85..913bf5fd68b3cecd5d9321b9527337f0163bd3f1 100644
--- a/ash/mus/test/wm_test_base.h
+++ b/ash/mus/test/wm_test_base.h
@@ -69,10 +69,6 @@ class WmTestBase : public testing::Test {
   void TearDown() override;
 
  private:
-  // Returns the RootWindowControllers ordered by display id (which we assume
-  // correlates with creation order).
-  std::vector<RootWindowController*> GetRootsOrderedByDisplayId();
-
   bool setup_called_ = false;
   bool teardown_called_ = false;
   std::unique_ptr<WmTestHelper> test_helper_;