hg: jdk/sandbox: 8218147: make_walkable asserts on multiple calls

[chris.hegarty at oracle.com]

Changeset: 1af5c0e68381
Author:    rehn
Date:      2019-04-17 09:25 +0200
URL:       http://hg.openjdk.java.net/jdk/sandbox/rev/1af5c0e68381

8218147: make_walkable asserts on multiple calls
Reviewed-by: dholmes, dcubed

! src/hotspot/share/jfr/periodic/sampling/jfrThreadSampler.cpp
! src/hotspot/share/runtime/thread.hpp