Enable lockless decoding of weak globals

Will help speed up decoding weak DexCache roots.

Change-Id: I9a68beb4106cbd383111a30e249c9b0149064e78
diff --git a/runtime/thread.cc b/runtime/thread.cc
index 63534b1..9929487 100644
--- a/runtime/thread.cc
+++ b/runtime/thread.cc
@@ -1728,7 +1728,7 @@
       result = nullptr;
     }
   } else if (kind == kGlobal) {
-    result = tlsPtr_.jni_env->vm->DecodeGlobal(const_cast<Thread*>(this), ref);
+    result = tlsPtr_.jni_env->vm->DecodeGlobal(ref);
   } else {
     DCHECK_EQ(kind, kWeakGlobal);
     result = tlsPtr_.jni_env->vm->DecodeWeakGlobal(const_cast<Thread*>(this), ref);