VM: Fix deadlock in time line recording.

runtime/vm/os_thread.h

Index: runtime/vm/os_thread.h
diff --git a/runtime/vm/os_thread.h b/runtime/vm/os_thread.h
index 9371b4538ae3d44227f23d13f58ac3bcd182551e..39eb815be469a68e8e0bff530d2679182d42f0bb 100644
--- a/runtime/vm/os_thread.h
+++ b/runtime/vm/os_thread.h
@@ -247,7 +247,7 @@ class OSThreadIterator : public ValueObject {
 
 class Mutex {
  public:
-  Mutex();
+  explicit Mutex(bool recursive = false);

[Ivan Posva, 2015/12/09 16:34:57]

The class Mutex is defined to not ever be recursive. If we have such a case then
the algorithm needs to be updated.

[Florian Schneider, 2015/12/10 14:44:21]

What would break otherwise? I didn't see any tests failing, so at least there is
no test coverage of this assumption.

   ~Mutex();
 
   void Lock();