Implement "create an element" when sync for Custom Element V1

third_party/WebKit/Source/core/dom/custom/CustomElement.cpp

Index: third_party/WebKit/Source/core/dom/custom/CustomElement.cpp
diff --git a/third_party/WebKit/Source/core/dom/custom/CustomElement.cpp b/third_party/WebKit/Source/core/dom/custom/CustomElement.cpp
index f1e231cb1aa15fc374b15bd984f335d3c4c34276..7f03f9417af00d6c2808ee473b05ba9d5f605dd0 100644
--- a/third_party/WebKit/Source/core/dom/custom/CustomElement.cpp
+++ b/third_party/WebKit/Source/core/dom/custom/CustomElement.cpp
@@ -100,7 +100,16 @@ HTMLElement* CustomElement::createCustomElement(Document& document, const Qualif
             if (flags & AsynchronousCustomElements)
                 return createCustomElementAsync(document, *definition, tagName);
 
-            // TODO(kojii): Synchronous mode implementation coming after async.
+            // When invoked from "create an element for a token":
+            // https://html.spec.whatwg.org/multipage/syntax.html#create-an-element-for-the-token
+            // 7. If this step throws an exception, then report the exception,
+            // and let element be instead a new element that implements
+            // HTMLUnknownElement.
+            if (flags & CreatedByParser)
+                return definition->createCustomElementIgnoringErrors(document, tagName);
+
+            // 6.1. If the synchronous custom elements flag is set:
+            return definition->createCustomElement(document, tagName);
         }
     }