Change Document::detach() to RELEASE_ASSERT all subframes are gone.

BUG=556724,577105

Review URL: https://codereview.chromium.org/1667573002

Cr-Commit-Position: refs/heads/master@{#373642}
void Document::pushCurrentScript(PassRefPtrWillBeRawPtr<HTMLScriptElement> newCurrentScript)
{
    ASSERT(newCurrentScript);
    m_currentScriptStack.append(newCurrentScript);
}

void Document::pushCurrentScript(PassRefPtrWillBeRawPtr<HTMLScriptElement> newCurrentScript)
{
    ASSERT(newCurrentScript);
    m_currentScriptStack.append(newCurrentScript);
}
