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

runtime/vm/compiler_stats.h

Index: runtime/vm/compiler_stats.h
diff --git a/runtime/vm/compiler_stats.h b/runtime/vm/compiler_stats.h
index 4487991224b94bcb84f543dcb677a05726a1cd14..b8894dc17d83519a5f40a87e811a5732f5fff2a6 100644
--- a/runtime/vm/compiler_stats.h
+++ b/runtime/vm/compiler_stats.h
@@ -73,10 +73,11 @@ class CompilerStats {
   char* PrintToZone();
 };
 
-// TODO(hausner): make the increment thread-safe.
 #define INC_STAT(thread, counter, incr)                                        \
   if (FLAG_compiler_stats) {                                                   \
-      (thread)->isolate()->compiler_stats()->counter += (incr); }
+    MutexLocker ml((thread)->isolate()->mutex());                              \
+    (thread)->isolate()->compiler_stats()->counter += (incr);                  \
+  }
 
 #define STAT_VALUE(thread, counter)                                            \
   ((FLAG_compiler_stats != false) ?                                            \