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::writeln(LocalDOMWindow* callingWindow, const Vector<String>& text, ExceptionState& exceptionState)
{
    ASSERT(callingWindow);
    StringBuilder builder;
    for (const String& string : text)
        builder.append(string);
    writeln(builder.toString(), callingWindow->document(), exceptionState);
}

void Document::writeln(LocalDOMWindow* callingWindow, const Vector<String>& text, ExceptionState& exceptionState)
{
    ASSERT(callingWindow);
    StringBuilder builder;
    for (const String& string : text)
        builder.append(string);
    writeln(builder.toString(), callingWindow->document(), exceptionState);
}
