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/gc_root-inl.h b/runtime/gc_root-inl.h
index 11ccd33..390ed3c 100644
--- a/runtime/gc_root-inl.h
+++ b/runtime/gc_root-inl.h
@@ -39,7 +39,7 @@
 
 template<class MirrorType>
 inline GcRoot<MirrorType>::GcRoot(ObjPtr<MirrorType, kIsDebugBuild> ref)
-    : GcRoot(ref.Decode()) { }
+    : GcRoot(ref.Ptr()) { }
 
 inline std::string RootInfo::ToString() const {
   std::ostringstream oss;