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::executeScriptsWaitingForResources()
{
    if (!isRenderingReady())
        return;
    if (ScriptableDocumentParser* parser = scriptableDocumentParser())
        parser->executeScriptsWaitingForResources();
}

void Document::executeScriptsWaitingForResources()
{
    if (!isRenderingReady())
        return;
    if (ScriptableDocumentParser* parser = scriptableDocumentParser())
        parser->executeScriptsWaitingForResources();
}
