Make CustomElementUpgradeSorterTest to use Element::AttachShadowRootInternal()

This patch changes |CustomElementUpgradeSorterTest| to use
|AttachShadowRootInternal()| instead of |Element::attachShadow()| to avoid using
script only functions for improving code health.

Change-Id: I625737366659a64ee26d4802c2972e1fd0488348
Reviewed-on: https://chromium-review.googlesource.com/1068537Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560978}

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

@@ -36,14 +36,6 @@ class CustomElementUpgradeSorterTest : public PageTestBase {
   ScriptState* GetScriptState() {
     return ToScriptStateForMainWorld(&GetFrame());
   }
-
-  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(CustomElementUpgradeSorterTest, inOtherDocument_notInSet) {
@@ -185,7 +177,7 @@ TEST_F(CustomElementUpgradeSorterTest, sorter_shadow) {
   Element* d = CreateElementWithId("a-a", "d");
 
   GetDocument().documentElement()->AppendChild(a);
-  ShadowRoot* s = AttachShadowTo(a);
+  ShadowRoot* s = &a->AttachShadowRootInternal(ShadowRootType::kOpen);
   a->AppendChild(d);
 
   s->AppendChild(b);