Get rid of unused member function AttachShadowTo() CustomElementRegistryTest class

This patch gets rid of unused member function |AttachShadowTo()| from
|CustomElementRegistryTest| class for improving code health.

Change-Id: I6eb81bb67c2b0521cd1c9c4fb554ab3ce71e9ea4
Reviewed-on: https://chromium-review.googlesource.com/1067563Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560975}

third_party/blink/renderer/core/html/custom/custom_element_registry_test.cc

@@ -45,14 +45,6 @@ class CustomElementRegistryTest : public PageTestBase {
                          HeapVector<Member<Element>>* elements) {
     Registry().CollectCandidates(desc, elements);
   }
-
-  ShadowRoot* AttachShadowTo(Element* element) {
-    NonThrowableExceptionState no_exceptions;
-    ShadowRootInit shadow_root_init;
-    shadow_root_init.setMode("open");
-    return element->attachShadow(GetScriptState(), shadow_root_init,
-                                 no_exceptions);
-  }
 };
 
 TEST_F(CustomElementRegistryTest,