Rename NullHandle to ScopedNullHandle

This makes it clearer that is invalid to do things like:
Handle<T> h = ScopedNullHandle<T>();

Bug: 26233305
Change-Id: I6d8f54eae01ec2e901cb7043afa853ea77db79fe
diff --git a/compiler/driver/compiler_driver.cc b/compiler/driver/compiler_driver.cc
index 56839f8..5630b08 100644
--- a/compiler/driver/compiler_driver.cc
+++ b/compiler/driver/compiler_driver.cc
@@ -898,8 +898,10 @@
           *dex_file,
           Runtime::Current()->GetLinearAlloc())));
       Handle<mirror::Class> klass(hs2.NewHandle(
-          class_linker->ResolveType(*dex_file, exception_type_idx, dex_cache,
-                                    NullHandle<mirror::ClassLoader>())));
+          class_linker->ResolveType(*dex_file,
+                                    exception_type_idx,
+                                    dex_cache,
+                                    ScopedNullHandle<mirror::ClassLoader>())));
       if (klass.Get() == nullptr) {
         const DexFile::TypeId& type_id = dex_file->GetTypeId(exception_type_idx);
         const char* descriptor = dex_file->GetTypeDescriptor(type_id);
diff --git a/compiler/driver/compiler_driver_test.cc b/compiler/driver/compiler_driver_test.cc
index 86f8b82..b6abc6e 100644
--- a/compiler/driver/compiler_driver_test.cc
+++ b/compiler/driver/compiler_driver_test.cc
@@ -149,9 +149,14 @@
   jobject class_loader;
   {
     ScopedObjectAccess soa(Thread::Current());
-    CompileVirtualMethod(NullHandle<mirror::ClassLoader>(), "java.lang.Class", "isFinalizable",
+    CompileVirtualMethod(ScopedNullHandle<mirror::ClassLoader>(),
+                         "java.lang.Class",
+                         "isFinalizable",
                          "()Z");
-    CompileDirectMethod(NullHandle<mirror::ClassLoader>(), "java.lang.Object", "<init>", "()V");
+    CompileDirectMethod(ScopedNullHandle<mirror::ClassLoader>(),
+                        "java.lang.Object",
+                        "<init>",
+                        "()V");
     class_loader = LoadDex("AbstractMethod");
   }
   ASSERT_TRUE(class_loader != nullptr);