Remove UserAgent ShadowRoot + Author ShadowRoot code.

third_party/WebKit/Source/core/dom/Element.cpp

Index: third_party/WebKit/Source/core/dom/Element.cpp
diff --git a/third_party/WebKit/Source/core/dom/Element.cpp b/third_party/WebKit/Source/core/dom/Element.cpp
index 427788b9f362641bf5e07718973f50e949ee0e74..35cce75ba86cd5db87310476a440e63dad92e279 100644
--- a/third_party/WebKit/Source/core/dom/Element.cpp
+++ b/third_party/WebKit/Source/core/dom/Element.cpp
@@ -1901,6 +1901,9 @@ ShadowRoot* Element::createShadowRoot(const ScriptState* scriptState, ExceptionS
             exceptionState.throwDOMException(InvalidStateError, "Shadow root cannot be created on a host which already hosts an user-agent shadow tree.");
             return nullptr;
         }
+    } else if (alwaysCreateUserAgentShadowRoot()) {
+        exceptionState.throwDOMException(InvalidStateError, "Shadow root cannot be created on a host which already hosts an user-agent shadow tree.");
+        return nullptr;
     }
     document().setShadowCascadeOrder(ShadowCascadeOrder::ShadowCascadeV0);