Fix JDWP race at runtime shutdown

When the runtime shuts down, it closes the JDWP connection with the
debugger. However, if a JDWP command is still being processed by the
JDWP handler thread when we close the connection, we won't be able to
send its reply.

Bug: 19628620
Change-Id: I20301325a347d66f3b9ef95ebe8f156abafb1f76
diff --git a/runtime/base/mutex.h b/runtime/base/mutex.h
index 6e7b04f..af00834 100644
--- a/runtime/base/mutex.h
+++ b/runtime/base/mutex.h
@@ -97,6 +97,7 @@
   kAllocTrackerLock,
   kDeoptimizationLock,
   kProfilerLock,
+  kJdwpShutdownLock,
   kJdwpEventListLock,
   kJdwpAttachLock,
   kJdwpStartLock,