Add a DCHECK in thread::GetPeer.

Motivated  by https://android-review.googlesource.com/#/c/333205/.

Test: test-art-host run-jdwp-test.sh
Change-Id: I173c060324aa0dc39144db55e3a97e672c012ba8
diff --git a/runtime/thread.h b/runtime/thread.h
index 3a1b7da..5dd6ae1 100644
--- a/runtime/thread.h
+++ b/runtime/thread.h
@@ -358,6 +358,7 @@
   uint64_t GetCpuMicroTime() const;
 
   mirror::Object* GetPeer() const REQUIRES_SHARED(Locks::mutator_lock_) {
+    DCHECK(Thread::Current() == this) << "Use GetPeerFromOtherThread instead";
     CHECK(tlsPtr_.jpeer == nullptr);
     return tlsPtr_.opeer;
   }