Bug 1398393 - Set Servo thread state on cooperative threads. r=billm

By doing this we avoid triggering assertions in the Servo code that ensure
we have registered the thread with Servo and set the proper state on it.

MozReview-Commit-ID: K6qHrYoQDLm

xpcom/threads/CooperativeThreadPool.cpp

@@ -8,6 +8,7 @@
 
 #include "base/message_loop.h"
 #include "mozilla/IOInterposer.h"
+#include "mozilla/ServoBindings.h"
 #include "nsError.h"
 #include "nsThreadUtils.h"