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/heap.h b/src/heap.h
index ffe2abf..ea4901d 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -100,8 +100,8 @@
   // Blocks the caller until the garbage collector becomes idle.
   static void WaitForConcurrentGcToComplete();
 
+  static uint32_t GetLockOwner(); // For SignalCatcher.
   static void Lock();
-
   static void Unlock();
 
   static const std::vector<Space*>& GetSpaces() {