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}
bool Document::queryCommandIndeterm(const String& commandName, ExceptionState& exceptionState)
{
    if (!isHTMLDocument() && !isXHTMLDocument()) {
        exceptionState.throwDOMException(InvalidStateError, "queryCommandIndeterm is only supported on HTML documents.");
        return false;
    }

    return command(this, commandName).state() == MixedTriState;
}

bool Document::queryCommandIndeterm(const String& commandName, ExceptionState& exceptionState)
{
    if (!isHTMLDocument() && !isXHTMLDocument()) {
        exceptionState.throwDOMException(InvalidStateError, "queryCommandIndeterm is only supported on HTML documents.");
        return false;
    }

    return command(this, commandName).state() == MixedTriState;
}
