More work for background compilation; move pending_functions_ from ObjectStore to Thread.

runtime/vm/compiler.h

Index: runtime/vm/compiler.h
diff --git a/runtime/vm/compiler.h b/runtime/vm/compiler.h
index 9c3e73fbf67595b66e6b7288fff4689a5c745bdf..791540a73b0fc020a7cc3bc0f2347ea2e0d34b24 100644
--- a/runtime/vm/compiler.h
+++ b/runtime/vm/compiler.h
@@ -119,12 +119,13 @@ class BackgroundCompiler : public ThreadPool::Task {
 
   void Add(const Function& f) const;
   RawFunction* RemoveOrNull() const;
+  RawFunction* LastOrNull() const;
 
   Isolate* isolate_;
-  bool running_;
-  bool* done_;
-  Monitor* monitor_;
-  Monitor* done_monitor_;
+  bool running_;       // While true, will try to read queue and compile.
+  bool* done_;         // True if the thread is done.
+  Monitor* queue_monitor_;  // Controls access to the queue.
+  Monitor* done_monitor_;   // Notify/wait that the thread is done.
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(BackgroundCompiler);
 };