Use a shared thread for media operations.

content/shell/renderer/webkit_test_runner.h

Index: content/shell/renderer/webkit_test_runner.h
diff --git a/content/shell/renderer/webkit_test_runner.h b/content/shell/renderer/webkit_test_runner.h
index 8191ed3a6cd746d90ae7fbd8b68f3b7a7d384471..eaa502ef2a2e44a6c344b74a3382d872cccb4bcc 100644
--- a/content/shell/renderer/webkit_test_runner.h
+++ b/content/shell/renderer/webkit_test_runner.h
@@ -9,6 +9,7 @@
 
 #include "base/files/file_path.h"
 #include "base/memory/scoped_ptr.h"
+#include "base/threading/thread.h"
 #include "content/public/renderer/render_view_observer.h"
 #include "content/public/renderer/render_view_observer_tracker.h"
 #include "content/shell/common/shell_test_configuration.h"
@@ -137,6 +138,7 @@ class WebKitTestRunner : public RenderViewObserver,
   bool focus_on_next_commit_;
 
   scoped_ptr<webkit_glue::TestMediaStreamClient> test_media_stream_client_;
+  base::Thread media_thread_;
 
   DISALLOW_COPY_AND_ASSIGN(WebKitTestRunner);
 };