Support base::test::ScopedTaskEnvironment and content::TestBrowserThreadBundle in the same scope.

content/public/test/test_browser_thread_bundle.h

 // Copyright 2013 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.
 
 // TestBrowserThreadBundle is a convenience class for creating a set of
 // TestBrowserThreads, a blocking pool, and a task scheduler in unit tests. For
 // most tests, it is sufficient to just instantiate the TestBrowserThreadBundle
 // as a member variable. It is a good idea to put the TestBrowserThreadBundle as
 // the first member variable in test classes, so it is destroyed last, and the
 // test threads always exist from the perspective of other classes.
@@ skipping 36 matching lines @@
 // REAL_IO_THREAD.
 //
 // For some tests it is important to emulate real browser startup. During real
 // browser startup, the main MessageLoop is created before other threads.
 // Passing DONT_CREATE_THREADS to constructor will delay creating other threads
 // until the test explicitly calls CreateThreads().
 //
 // DONT_CREATE_THREADS should only be used when the options specify at least
 // one real thread other than the main thread.
 //
+// TestBrowserThreadBundle may be instantiated in a scope where there is already
+// a base::test::ScopedTaskEnvironment. In that case, it will use the
+// MessageLoop and the TaskScheduler provided by this
+// base::test::ScopedTaskEnvironment instead of creating its own. The ability to
+// have a base::test::ScopedTaskEnvironment and a TestBrowserThreadBundle in the
+// same scope is useful when a fixture that inherits from a fixture that
+// provides a base::test::ScopedTaskEnvironment needs to add support for browser
+// threads.
+//
 // Basic usage:
 //
 //   class MyTestFixture : public testing::Test {
 //    public:
 //     (...)
 //
 //    protected:
 //     // Must be the first member (or at least before any member that cares
 //     // about tasks) to be initialized first and destroyed last. protected
 //     // instead of private visibility will allow controlling the task
@@ skipping 62 matching lines @@
 
   int options_;
   bool threads_created_;
 
   DISALLOW_COPY_AND_ASSIGN(TestBrowserThreadBundle);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_TEST_TEST_BROWSER_THREAD_BUNDLE_H_