Fixes clang warning in GLSurfaceOzone.

Introduced in https://codereview.chromium.org/938873002/

R=piman@chromium.org
BUG=None

Review URL: https://codereview.chromium.org/967343003

Cr-Commit-Position: refs/heads/master@{#318826}
diff --git a/ui/gl/gl_surface_ozone.cc b/ui/gl/gl_surface_ozone.cc
index f24708c3..47ea8d6 100644
--- a/ui/gl/gl_surface_ozone.cc
+++ b/ui/gl/gl_surface_ozone.cc
@@ -305,6 +305,8 @@
     }
 
    private:
+    ~SurfaceImage() override {}
+
     scoped_refptr<ui::NativePixmap> pixmap_;
   };