Fix mojo::shell::Context shutdown.

Make shutdown explicit (i.e., if you call Init(), you have to call
Shutdown()).

R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/939353002
diff --git a/shell/shell_test_base.cc b/shell/shell_test_base.cc
index 6e4f18c..20e6b35 100644
--- a/shell/shell_test_base.cc
+++ b/shell/shell_test_base.cc
@@ -24,10 +24,14 @@
 }
 
 void ShellTestBase::SetUp() {
-  shell_context_.Init();
+  CHECK(shell_context_.Init());
   SetUpTestApplications();
 }
 
+void ShellTestBase::TearDown() {
+  shell_context_.Shutdown();
+}
+
 ScopedMessagePipeHandle ShellTestBase::ConnectToService(
     const GURL& application_url,
     const std::string& service_name) {