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::modifiedStyleSheet(StyleSheet* sheet, StyleResolverUpdateMode updateMode)
{
    if (isActive())
        styleEngine().modifiedStyleSheet(sheet);
    styleResolverChanged(updateMode);
}

void Document::modifiedStyleSheet(StyleSheet* sheet, StyleResolverUpdateMode updateMode)
{
    if (isActive())
        styleEngine().modifiedStyleSheet(sheet);
    styleResolverChanged(updateMode);
}
