Add DisplayCompositorTest.

services/ui/surfaces/display_compositor.h

Index: services/ui/surfaces/display_compositor.h
diff --git a/services/ui/surfaces/display_compositor.h b/services/ui/surfaces/display_compositor.h
index d31ccfdfd19aaf5e6655bae09339120a698587c9..568f4d393fb1505a699bca5fe8a2a15395f778db 100644
--- a/services/ui/surfaces/display_compositor.h
+++ b/services/ui/surfaces/display_compositor.h
@@ -7,6 +7,10 @@
 
 #include <stdint.h>
 
+#include <memory>
+#include <unordered_map>
+#include <vector>
+
 #include "base/macros.h"
 #include "cc/ipc/display_compositor.mojom.h"
 #include "cc/surfaces/frame_sink_id.h"
@@ -31,6 +35,10 @@ class SurfaceManager;
 
 namespace ui {
 
+namespace test {
+class DisplayCompositorTest;
+}
+
 class DisplayCompositorClient;
 class GpuCompositorFrameSink;
 class MusGpuMemoryBufferManager;
@@ -78,6 +86,9 @@ class DisplayCompositor : public cc::SurfaceObserver,
       bool destroy_compositor_frame_sink);
 
  private:
+  friend class test::DisplayCompositorTest;
+
+  const cc::SurfaceId& GetRootSurfaceId() const;
 
   // cc::SurfaceObserver implementation.
   void OnSurfaceCreated(const cc::SurfaceId& surface_id,
@@ -99,6 +110,10 @@ class DisplayCompositor : public cc::SurfaceObserver,
   gpu::ImageFactory* image_factory_;
   cc::mojom::DisplayCompositorClientPtr client_;
 
+  // Will normally point to |manager_| as it provides the interface. For tests
+  // it will be swapped out with a mock implementation.
+  cc::SurfaceReferenceManager* reference_manager_;
+
   // SurfaceIds that have temporary references from top level root so they
   // aren't GC'd before DisplayCompositorClient can add a real reference. This
   // is basically a collection of surface ids, for example: