Fail threads attaching during runtime shutdown.

Introduce counters to indicate that threads are being born. Don't allow
thread birth to occur during runtime shutdown.

Bug: 7000936

Change-Id: Ib0d78f78c0ff126a4b5d3b5a6f1a2ff8f5061ae9
diff --git a/src/jdwp/jdwp_main.cc b/src/jdwp/jdwp_main.cc
index a09c488..0c3a9a0 100644
--- a/src/jdwp/jdwp_main.cc
+++ b/src/jdwp/jdwp_main.cc
@@ -284,7 +284,7 @@
 
 void JdwpState::Run() {
   Runtime* runtime = Runtime::Current();
-  runtime->AttachCurrentThread("JDWP", true, runtime->GetSystemThreadGroup());
+  CHECK(runtime->AttachCurrentThread("JDWP", true, runtime->GetSystemThreadGroup()));
 
   VLOG(jdwp) << "JDWP: thread running";