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This patch fixes ScriptCustomElementDefinition::createElementSync to
create an element with its ownerDocument set to the given document by
using the construction stack.

Before this fix, V8HTMLElement::constructorCustom assumed it matches to
window.document. The assumption does not stand when custom elements are
created in import documents.

BUG=640465

Review-Url: https://codereview.chromium.org/2274883005
Cr-Commit-Position: refs/heads/master@{#414434}