Eliminate mojo::Shell client lib class

mojo/shell/background/tests/test_service.cc

Index: mojo/shell/background/tests/test_service.cc
diff --git a/mojo/shell/background/tests/test_service.cc b/mojo/shell/background/tests/test_service.cc
index 4d001afe1b82ebcaaf15afe0fe7cc804a91b84fa..8b68c59aebcf00ded32de33e2f01291536e9aace 100644
--- a/mojo/shell/background/tests/test_service.cc
+++ b/mojo/shell/background/tests/test_service.cc
@@ -21,10 +21,6 @@ class TestClient : public ShellClient,
 
  private:
   // ShellClient:
-  void Initialize(Shell* shell,
-                  const std::string& url,
-                  uint32_t id,
-                  uint32_t user_id) override {}
   bool AcceptConnection(Connection* connection) override {
     connection->AddInterface(this);
     return true;