Fix a heap lock/thread list lock deadlock.

We had an uncaught OOME whose uncaught exception handler -- running
with the thread lock held -- was trying to cause a GC while some
other thread had the heap lock and was waiting for the thread list
lock.

Change-Id: I22177129562268837127d9edcc63ef5e93054bdf
diff --git a/src/thread_list.h b/src/thread_list.h
index 35b23e7..f5e1a3a 100644
--- a/src/thread_list.h
+++ b/src/thread_list.h
@@ -32,6 +32,7 @@
   ~ThreadList();
 
   void Dump(std::ostream& os);
+  uint32_t GetLockOwner(); // For SignalCatcher.
 
   // Thread suspension support.
   void FullSuspendCheck(Thread* thread);