Fix MediaDevices* tests dependency on BrowserThread globals after ~BrowserThreadImpl().

Mostly a matter of tweaking destruction order in test and adding an
additional flush instead of depending on ~MediaStreamManager() implicitly
flushing MediaStreamManager::video_capture_thread_().

This is a prereq to https://codereview.chromium.org/2464233002 which
will from now on reset BrowserThread globals associated with destroyed
BrowserThreadImpls and thus makes DCHECK_CURRENTLY_ON fail after that
fact.

BUG=653916

Review-Url: https://codereview.chromium.org/2535043003
Cr-Commit-Position: refs/heads/master@{#435055}

content/browser/renderer_host/media/media_devices_dispatcher_host_unittest.cc

@@ -251,9 +251,14 @@ class MediaDevicesDispatcherHostTest : public testing::Test {
     }
   }
 
-  std::unique_ptr<MediaDevicesDispatcherHost> host_;
+  // The order of these members is important on teardown:
+  // MediaDevicesDispatcherHost expects to be destroyed on the IO thread while
+  // MediaStreamManager expects to be destroyed after the IO thread has been
+  // uninitialized.
   std::unique_ptr<MediaStreamManager> media_stream_manager_;
   content::TestBrowserThreadBundle thread_bundle_;
+  std::unique_ptr<MediaDevicesDispatcherHost> host_;
+
   std::unique_ptr<media::AudioManager, media::AudioManagerDeleter>
       audio_manager_;
   content::TestBrowserContext browser_context_;
@@ -326,6 +331,13 @@ TEST_F(MediaDevicesDispatcherHostTest, EnumerateAllDevicesUniqueOrigin) {
       base::Bind(&MediaDevicesDispatcherHostTest::ValidOriginCallback,
                  base::Unretained(this)));
   base::RunLoop().RunUntilIdle();
+#if defined(OS_WIN)
+  // On Windows, the underlying MediaStreamManager uses a separate thread for
+  // video capture which must be flushed to guarantee that the callback bound to
+  // EnumerateDevices above is invoked before the end of this test's body.
+  media_stream_manager_->FlushVideoCaptureThreadForTesting();
+  base::RunLoop().RunUntilIdle();
+#endif
 }
 
 };  // namespace content

content/browser/renderer_host/media/media_devices_manager_unittest.cc

@@ -125,8 +125,8 @@ class MockMediaDeviceChangeSubscriber : public MediaDeviceChangeSubscriber {
 class MediaDevicesManagerTest : public ::testing::Test {
  public:
   MediaDevicesManagerTest()
-      : video_capture_device_factory_(nullptr),
-        thread_bundle_(content::TestBrowserThreadBundle::IO_MAINLOOP) {}
+      : thread_bundle_(content::TestBrowserThreadBundle::IO_MAINLOOP),
+        video_capture_device_factory_(nullptr) {}
   ~MediaDevicesManagerTest() override {}
 
   MOCK_METHOD1(MockCallback, void(const MediaDeviceEnumeration&));
@@ -156,10 +156,13 @@ class MediaDevicesManagerTest : public ::testing::Test {
         type, MediaDevicesManager::CachePolicy::SYSTEM_MONITOR);
   }
 
+  // Must outlive MediaDevicesManager as ~MediaDevicesManager() verifies it's
+  // running on the IO thread.
+  TestBrowserThreadBundle thread_bundle_;
+
   std::unique_ptr<MediaDevicesManager> media_devices_manager_;
   scoped_refptr<VideoCaptureManager> video_capture_manager_;
   MockVideoCaptureDeviceFactory* video_capture_device_factory_;
-  TestBrowserThreadBundle thread_bundle_;
   std::unique_ptr<MockAudioManager, media::AudioManagerDeleter> audio_manager_;
 
  private:

content/browser/renderer_host/media/media_stream_manager.cc

@@ -1757,4 +1757,10 @@ void MediaStreamManager::SetGenerateStreamCallbackForTesting(
   generate_stream_test_callback_ = test_callback;
 }
 
+#if defined(OS_WIN)
+void MediaStreamManager::FlushVideoCaptureThreadForTesting() {
+  video_capture_thread_.FlushForTesting();
+}
+#endif
+
 }  // namespace content

content/browser/renderer_host/media/media_stream_manager.h

@@ -249,6 +249,10 @@ class CONTENT_EXPORT MediaStreamManager
   void SetGenerateStreamCallbackForTesting(
       GenerateStreamTestCallback test_callback);
 
+#if defined(OS_WIN)
+  void FlushVideoCaptureThreadForTesting();
+#endif
+
  private:
   // Contains all data needed to keep track of requests.
   class DeviceRequest;