Rename ObjPtr::Decode to ObjPtr::Ptr

Done to prevent ambiguity with ScopedObjectAccess::Decode.

Bug: 31113334

Test: test-art-host
Change-Id: I07a2497cc9cf66386311798933547471987fc316
diff --git a/runtime/native/java_lang_ref_Reference.cc b/runtime/native/java_lang_ref_Reference.cc
index 9a088ed..95f6d51 100644
--- a/runtime/native/java_lang_ref_Reference.cc
+++ b/runtime/native/java_lang_ref_Reference.cc
@@ -29,7 +29,7 @@
   ScopedFastNativeObjectAccess soa(env);
   ObjPtr<mirror::Reference> ref = soa.Decode<mirror::Reference>(javaThis);
   mirror::Object* const referent =
-      Runtime::Current()->GetHeap()->GetReferenceProcessor()->GetReferent(soa.Self(), ref.Decode());
+      Runtime::Current()->GetHeap()->GetReferenceProcessor()->GetReferent(soa.Self(), ref.Ptr());
   return soa.AddLocalReference<jobject>(referent);
 }