Don't report down-calls as unhandled exceptions.

Bug: 15310540
Also, narrow scope of catch/deoptimize stack visitors that are specific to
quick exception delivery.

Change-Id: Ib13a006ce1347acb93a36b0186550d4c3ec2034b
diff --git a/runtime/thread.h b/runtime/thread.h
index 4601248..88b4b0d 100644
--- a/runtime/thread.h
+++ b/runtime/thread.h
@@ -1090,6 +1090,7 @@
   friend class Dbg;  // For SetStateUnsafe.
   friend class gc::collector::SemiSpace;  // For getting stack traces.
   friend class Runtime;  // For CreatePeer.
+  friend class QuickExceptionHandler;  // For dumping the stack.
   friend class ScopedThreadStateChange;
   friend class SignalCatcher;  // For SetStateUnsafe.
   friend class StubTest;  // For accessing entrypoints.