Split out the extensions gin::Runner implementation into a separate class.

extensions/renderer/script_context_unittest.cc

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "extensions/renderer/module_system_test.h"
+#include "extensions/renderer/script_context.h"
+#include "gin/per_context_data.h"
+#include "gin/runner.h"
+
+namespace extensions {
+
+using ScriptContextTest = ModuleSystemTest;
+
+TEST_F(ScriptContextTest, GinRunnerLifetime) {
+  ExpectNoAssertionsMade();
+  base::WeakPtr<gin::Runner> weak_runner =
+      gin::PerContextData::From(env()->context()->v8_context())
+          ->runner()
+          ->GetWeakPtr();
+  env()->ShutdownModuleSystem();
+  EXPECT_FALSE(weak_runner);
+}
+
+}  // namespace extensions