Clean up Handle usage.

Prefer using ConstHandle instead of Handle as function arguments
since you can't assign new references to ConstHandle which helps
prevent bugs.

Changed NullHandle to be a ConstHandle so that you can never modify
it to be a non null reference.

Change-Id: I81cb979f6f8d5b49e5614966a2caf28c3701dd4f
diff --git a/compiler/common_compiler_test.h b/compiler/common_compiler_test.h
index df06b71..4e74f0a 100644
--- a/compiler/common_compiler_test.h
+++ b/compiler/common_compiler_test.h
@@ -63,11 +63,11 @@
 
   void CompileMethod(mirror::ArtMethod* method) SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
 
-  void CompileDirectMethod(Handle<mirror::ClassLoader> class_loader, const char* class_name,
+  void CompileDirectMethod(ConstHandle<mirror::ClassLoader> class_loader, const char* class_name,
                            const char* method_name, const char* signature)
       SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
 
-  void CompileVirtualMethod(Handle<mirror::ClassLoader> class_loader, const char* class_name,
+  void CompileVirtualMethod(ConstHandle<mirror::ClassLoader> class_loader, const char* class_name,
                             const char* method_name, const char* signature)
       SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);