Fix preserve backbuffer with native GMBs.

gpu/command_buffer/tests/gl_manager.h

Index: gpu/command_buffer/tests/gl_manager.h
diff --git a/gpu/command_buffer/tests/gl_manager.h b/gpu/command_buffer/tests/gl_manager.h
index 4dff34d72ef5ae8576987ff94a769887215c6ff4..aa8a49acb0812f2316c17e770b2d937fdfa29b33 100644
--- a/gpu/command_buffer/tests/gl_manager.h
+++ b/gpu/command_buffer/tests/gl_manager.h
@@ -82,6 +82,8 @@ class GLManager : private GpuControl {
     bool backbuffer_alpha;
     // The ImageFactory to use to generate images for the backbuffer.
     gpu::ImageFactory* image_factory;
+    // Whether to preserve the backbuffer after a call to SwapBuffers().
+    bool preserve_backbuffer;
   };
   GLManager();
   ~GLManager() override;